Hmm... they should not be able to register. I bet they are clearing cookies... >:
If you want to use multiple names replace this:
if (checkname == "[his last used username]")
with this:
if ((checkname == "[his last used username]") || (checkname == "[second username]") || (checkname == "[third username]"))
You can use the same piece of javascript with IP numbers if you use SSI to obtain IP numbers.