Anonimous could vote only for 3 nodes in rating

Hello! I have a stusent web-site where I use vote up/down for university rating. So, yesterday some students decided to "help" their university to be on top. They started upping their university, and downing others university. I cleared all spam votes, but I`d liked to have such function - it would be great if anonimous user could vote only for 3 (or something) node on all rating. I think, it could reduce risks of spamming results.

Well, I`m not a programmer, I couldn`t do it myself, but it seems to me that it should be mysql request. If some IP vote more than 3 times, all other votes from that IP are deleted. How could I do that?

I use Vote up/down and VotingAPI and I don`t know for what module it should be a request, so I copy request to both.

P.S. Sorry for mistakes, I`m not native English. Thanks for help!!!