This should come in pretty handy, I think, it's a PHP script that blocks specified IPs from viewing yoru ads (or anything else you put into the PHP include), it should save a lot trouble if you're certain a specific IP is responsible for click spamming... http://www.obli.net/item/295
Well, you could use it for your own IP, or like the guy who posted it, if you're paranoid about a shared IP that you frequent (his school's IP, in that case) Hell, if you could find your competitor's IP it could go into the blocker as well...