How about a timed kick if they don't change their nick after a minute or 2?This should do the trick (untested):

on *:KICK:#:.hdel guest $nick
on @*:JOIN:#: {
  if (%aguest) && ($regex($nick,/\A(guest\d{5})/i)) && (!$hget(time,$nick)) { 
    hadd -mz guest $nick 120
    .msg $chan $nick $+ , you have about 2 minutes to change your nick!
    .timercheck 1 120 kickguest
  }
  else { ban -ku300 $chan $nick 5 Change your nick before returning! }
}
alias -l kickguest {
  var %x = 1
  while ($hget(guest,%x).item) {
    if ($v1 ison $chan) {
      ban -ku300 $chan $v1 5 Failed to change nick!
    }
    inc %x
  }
}
menu menubar,channel {
  $iif(%aguest != $false,[On],[Off]) Anti Guests:{
    set %aguest $iif(%aguest == $false,$true,$false)
    echo -a  4Anti Guests kick is now $iif(%aguest == $true,Enabled,Disabled)
  }
}