heres a beter solution, not to drive anyone awa. but if you have access to your sites cpanel (which you should) go there and hopefully you have the ability to create custom errors. Well go into and edit the 403 error one, this is what shows when someone is blocked fro ma website. and place this in there. if you do this, if you ban the persons IP from your website rather than the forums they will be redirected no matter what unless they get smart and use a proxy.
Code:
<meta HTTP-EQUIV="REFRESH" content="0; url=http://www.theybannedme.com">