[Lynne]

Actually, I don't think it's the registration code you need to look at, it's the code from when you ban them. At that point, you would need to change the cache to remove their name if it was the last registered user. So, I'd start by looking first at the ban code (modcp/banning.php ?) and seeing if there is a plugin to use and then look at the register.php page to see where the last username is cached so you can steal that code.

Have you tried a search in the modifications forums? I thought I had seen a mod to do this before.