The easiest way would be to use an access list.

Code:
on *:text:*:?:{
  if ($level($address($nick,2)) < 5) { msg $nick You're not authorized to message me. | /closemsg $nick | /echo -a  $nick tried to message you.  }
}

menu nicklist {
  -
  $iif($level($address($$1,2)) >= 5,Remove Message Access):{ /ruser 5 $address($$1,2) | /echo -a Removed $$1 from message access. }
  $iif($level($address($$1,2)) < 5,Add Message Access):{ /auser 5 $address($$1,2) | /echo -a Added $$1 to message access. }
}