Use getenv("REMOTE_ADDR") to get his IP Address and save it to a file or in SQL, then do an ip address check (query the file or SQL) when people enter your site, if ip is found, send him to somewhere else using header("Location: http://somewhereElse.com");.
You can't block him if he uses dynamic ip addressing, but you can get his ip address and notify his ISP... If you use Windows, use the tracert command tracert 127.0.0.1 (replace 127.0.0.1 with his ip address)
And if you want to harass him too, I wrote a little script that'll send as many e-mails as you specify in the configuration, and the recipient can not block the sender's address because it uses random characters as sender's address. This script (evilMail) can be found here, that ought to annoy the xxxx out of him.