Add usermode +q which requires users /msg'ing or /notice'ing you to be in at least one common channel.
This is designed to stop the spam bots which sit outside a channel, while a spy sits inside, preventing channel operators dealing with the problem.
We currently also block invites, this might not be such a good idea, but these days everyone can get Q.

diff -r 0543ca28447d include/channel.h
--- a/include/channel.h
+++ b/include/channel.h
@@ -462,5 +462,6 @@
 extern void free_ban(struct Ban *ban);
 
 extern unsigned int get_channel_marker(void);
+extern int common_chan_count(struct Client *a, struct Client *b, int max);
 
 #endif /* INCLUDED_channel_h */
diff -r 0543ca28447d include/client.h
--- a/include/client.h
+++ b/include/client.h
@@ -90,7 +90,7 @@
 #define FlagClr(set,flag) ((set)->bits[FLAGSET_INDEX(flag)] &= ~FLAGSET_MASK(flag))
 
 /** String containing valid user modes, in no particular order. */
-#define infousermodes "dioOswkghxRXInP"
+#define infousermodes "dioOswkghxRXInPq"
 
 /** Character to indicate no oper name available */
 #define NOOPERNAMECHARACTER '-'
@@ -191,6 +191,8 @@
     FLAG_NOCHAN,                    /**< user's channels are hidden */
     FLAG_NOIDLE,                    /**< user's idletime is hidden */
     FLAG_XTRAOP,                    /**< oper has special powers */
+    FLAG_COMMONCHANSONLY,           /**< SNIRCD_q: hide privmsgs/notices if in no
+					 common channels (with +ok exceptions) */
 
     FLAG_LAST_FLAG,                 /**< number of flags */
     FLAG_LOCAL_UMODES = FLAG_LOCOP, /**< First local mode flag */
@@ -623,6 +625,8 @@
 #define IsParanoid(x)           HasFlag(x, FLAG_PARANOID)
 /** Return non-zero if the server should send opername information */
 #define IsSendOperName(x)         HasFlag(x, FLAG_OPERNAME)
+/** Return non-zero if the client has set mode +q (common chans only). */
+#define IsCommonChansOnly(x)    HasFlag(x, FLAG_COMMONCHANSONLY)
 
 /** Return non-zero if the client has operator or server privileges. */
 #define IsPrivileged(x)         (IsAnOper(x) || IsServer(x))
@@ -689,6 +693,8 @@
 #define SetAccountOnly(x)       SetFlag(x, FLAG_ACCOUNTONLY)
 /** Mark a client as having mode +P (paranoid). */
 #define SetParanoid(x)          SetFlag(x, FLAG_PARANOID)
+/** Mark a client as having mode +q (common chans only). */
+#define SetCommonChansOnly(x)   SetFlag(x, FLAG_COMMONCHANSONLY)
 
 /** Return non-zero if \a sptr sees \a acptr as an operator. */
 #define SeeOper(sptr,acptr) (IsAnOper(acptr) && (HasPriv(acptr, PRIV_DISPLAY) \
@@ -738,6 +744,8 @@
 #define ClearAccountOnly(x)     ClrFlag(x, FLAG_ACCOUNTONLY)
 /** Remove mode +P (paranoid) from a client */
 #define ClearParanoid(x)        ClrFlag(x, FLAG_PARANOID)
+/** Remove mode +q (common chans only) from a client */
+#define ClearCommonChansOnly(x) ClrFlag(x, FLAG_COMMONCHANSONLY)
 
 /* free flags */
 #define FREEFLAG_SOCKET	0x0001	/**< socket needs to be freed */
diff -r 0543ca28447d include/numeric.h
--- a/include/numeric.h
+++ b/include/numeric.h
@@ -420,6 +420,7 @@
 /*	ERR_HTMDISABLED      486	   unreal */
 #define ERR_ACCOUNTONLY      486	/* QuakeNet/ASUKA extension */
 /*      ERR_CHANTOORECENT    487           IRCnet extension (?) */
+#define ERR_COMMONCHANSONLY  487        /* QuakeNet/snircd extension */
 /*      ERR_TSLESSCHAN       488           IRCnet extension (?) */
 #define ERR_VOICENEEDED      489        /* Undernet extension */
 
diff -r 0543ca28447d ircd/channel.c
--- a/ircd/channel.c
+++ b/ircd/channel.c
@@ -3819,3 +3819,39 @@
 
   return marker;
 }
+
+/* Returns the number of common channels between two users, upto max. */
+int common_chan_count(struct Client *a, struct Client *b, int max)
+{
+  int count = 0;
+  struct Membership *cptr;
+  struct User *ua, *ub;
+  unsigned int marker = get_client_marker();
+
+  ua = cli_user(a);
+  ub = cli_user(b);
+
+  /* makes no difference to the big O complexity I know */
+  if(ua->joined > ub->joined)
+  {
+    struct User *swapee = ua;
+    ua = ub;
+    ub = swapee;
+  }
+
+  for (cptr=ua->channel;cptr;cptr=cptr->next_channel)
+  {
+    cptr->channel->marker = marker;
+  }
+
+  for (cptr=ub->channel;cptr;cptr=cptr->next_channel)
+  {
+    if (cptr->channel->marker == marker) {
+      count++;
+      if (max && (count >= max))
+        return count;
+    }
+  }
+
+  return count;
+}
diff -r 0543ca28447d ircd/ircd_relay.c
--- a/ircd/ircd_relay.c
+++ b/ircd/ircd_relay.c
@@ -310,6 +310,10 @@
   if (IsAccountOnly(acptr) && !IsAccount(sptr) && !IsXtraOp(sptr))
     return;
 
+  /* slug: same applies here, since only opers can be +k */
+  if (IsCommonChansOnly(acptr) && !IsXtraOp(sptr) && !common_chan_count(acptr, sptr, 1))
+    return;
+
   if (!(is_silenced(sptr, acptr)))
     sendcmdto_one(sptr, CMD_PRIVATE, acptr, "%s :%s", name, text);
 }
@@ -362,6 +366,9 @@
   if (IsAccountOnly(acptr) && !IsAccount(sptr) && !IsXtraOp(sptr))
     return;
 
+  if (IsCommonChansOnly(acptr) && !IsXtraOp(sptr) && !common_chan_count(acptr, sptr, 1))
+    return;
+
   if (!(is_silenced(sptr, acptr)))
     sendcmdto_one(sptr, CMD_NOTICE, acptr, "%s :%s", name, text);
 }
@@ -401,6 +408,11 @@
     return;
   }
 
+  if (IsCommonChansOnly(acptr) && !IsXtraOp(sptr) && !common_chan_count(acptr, sptr, 1)) {
+    send_reply(sptr, ERR_COMMONCHANSONLY, cli_name(acptr));
+    return;
+  }
+
   /*
    * send away message if user away
    */
@@ -445,6 +457,9 @@
   if (IsAccountOnly(acptr) && !IsAccount(sptr) && !IsXtraOp(sptr))
     return;
 
+  if (IsCommonChansOnly(acptr) && !IsXtraOp(sptr) && !common_chan_count(acptr, sptr, 1))
+    return;
+
   /*
    * deliver the message
    */
diff -r 0543ca28447d ircd/m_invite.c
--- a/ircd/m_invite.c
+++ b/ircd/m_invite.c
@@ -171,6 +171,9 @@
     return 0;
   }
   
+  if (IsCommonChansOnly(acptr) && !IsXtraOp(sptr) && !common_chan_count(acptr, sptr, 1))
+    return;
+
   if (check_target_limit(sptr, acptr, cli_name(acptr), 0))
     return 0;
 
diff -r 0543ca28447d ircd/s_err.c
--- a/ircd/s_err.c
+++ b/ircd/s_err.c
@@ -1006,7 +1006,7 @@
 /* 486 */
   { ERR_ACCOUNTONLY, "%s :You must be authed in order to message this user -- For details of how to obtain an account visit %s", "486" },
 /* 487 */
-  { 0 },
+  { ERR_COMMONCHANSONLY, "%s :You must share at least one channel with this user in order to message them", "487" },
 /* 488 */
   { 0 },
 /* 489 */
diff -r 0543ca28447d ircd/s_user.c
--- a/ircd/s_user.c
+++ b/ircd/s_user.c
@@ -540,7 +540,8 @@
   { FLAG_NOCHAN,      'n' },
   { FLAG_NOIDLE,      'I' },
   { FLAG_SETHOST,     'h' },
-  { FLAG_PARANOID,    'P' }
+  { FLAG_PARANOID,    'P' },
+  { FLAG_COMMONCHANSONLY, 'q' }
 };
 
 /** Length of #userModeList. */
@@ -851,7 +852,9 @@
       send_reply(source, ERR_ACCOUNTONLY, cli_name(dest), feature_str(FEAT_URLREG));
     return 0;
   }
-          
+
+  /* No check here for IsCommonChansOnly since by definition we share at least one! */
+
   if (is_notice)
     sendcmdto_one(source, CMD_NOTICE, dest, "%C :%s", dest, text);
   else
@@ -1319,6 +1322,12 @@
         else
           ClearParanoid(sptr);
 	break;
+      case 'q':
+	if (what == MODE_ADD)
+          SetCommonChansOnly(sptr);
+        else
+          ClearCommonChansOnly(sptr);
+	break;
       case 'r':
 	if ((what == MODE_ADD) && *(p + 1)) {
 	  account = *(++p);