Code:
on ^*:QUIT: {
  if ($len($1-) > 50) { 
    var %a = 1
    while ($comchan($nick,%a)) {
      echo $colour(quit) -i2 $v1 *** $nick has quit IRC (quit message too long)
      inc %a
    }
  haltdef
  }
}


You can change the 50 to whatever you deem appropriate, or change it to if ($nick = certain person) instead

change the echo to whatever you wish, that echo is mIRC's default... with the quit message changed...

-Rappy (routed)