If you don't wanna make the guest feel unwanted by your ban, and to make things seem more like their computer is messed up, or the site was down, I highly recommend installing the 404-ban error method, page not found, instead of using vBulletin's ban.
It's a great method of ban anyway, in general.
In functions.php, just find every occurence of:
PHP Code:
eval("standarderror(\"" . gettemplate("error_banip") ."\");");
And replace it with:
PHP Code:
header("HTTP/1.0 404 Not Found");
header("Status: 404 Not Found");
exit;
This will output the 404 message relative to their browser, also. (unless the server has its own)

[high]* Velocd laughs cynically

[/high]