Use the ^ prefix with the haltdef command.
on ^!*:JOIN:#: {
if nick is clone {
haltdef
Replace join message
}
}
But do your own routine for clone checking as that one isn't real. Also, the ! prefix prevents it from being triggered when you join.
You could do...
on ^!*:JOIN:#: {
if ($ialchan($mask($fulladdress,2),#,2)) {
haltdef
Replace join message
}
}
Also you could replace
$mask($fulladdress,2) with
$wildsite