You could try autorank php, its pretty good. I've also just developed one(its free). It not as feature rich as autorank but take a look.
http://www.chipmunk-scripts.com/topsites .
My script uses an anti-cheat gateway and IP tracking.
The truth of the matter is, if they clean cookies and proximize their IP, there is no way to prevent cheating. I've tried it on many topsite system and if you do those two, its impossible to track you.