Someone can easily mask their IP if they load a proxy. One can easily find working anonymous proxies, like this site: http://www.samair.ru/proxy/ they're all over the net.

even if you ban an IP, if someone loads a proxy you have to ban them again. The banning technique would need to be more tricky. Maybe set a cookie or something - but they could still clear those.

lets complete the rack & pinion deal, then we can test this -l8er