Heh, I've been doing this for the last 4 months on my vBulletin, but I failed at emulating the 404-error page in HTML, so instead I simply copy+pasted an image of the 404-error page into Adobe, saved it as a really light-weight GIF, then edited the ban-template in the ACP and set the image using CSS as a background of the page, but so it does not repeat. It works well, but if the user simply tried to highlight any of the 404-error text, they would find out it would not be possible.. though, that doesn't mean they'd figure out what I had done to ban them
Anyway, your method wins over mine, so I guess it's time for upgrade
[high]* Velocd clicks install[/high]