If you are ABSOLUTELY sure he always uses the same IP, do something like this :
<?
if($_SERVER["REMOTE_ADDR"] == "abc.def.ghi.jkl") {
exit("Sorry, you're banned! Mouahahahahahahah!<BR/>\n");
}
?>
... where abc.def.ghi.jkl is the IP address ...
Also, place it at the top of the page(s).