I've discovered 2 cheaters on a contest: What should I do?

I'm really in need of some advice. I participate in a lot of icontests and I've recently discovered 2 users are cheating by asking other people to vote for them and possibly even creating accounts to pad the voting. I'm very upset because the 2 of them have 'won' quite a few rounds in this manner, beating out much better icons over and over. I informed the person who ran the contest, leaving it up to them what to do. They posted a reminder on the forum about the rules against cheating. It seemed to work- for a split second. Then they just went back to cheating.

So now I'm debating on whether or not I should take further action. These two users are nice enough- other than the cheating of course -and I don't particularly want to make enemies of them. However, what they're doing is totally unfair to everyone else participating in the contest.

I don't want to publicly humiliate them by pointing fingers [although I think it's pretty obvious to everyone what they're doing].

Should I send them a private message confronting them and asking them to stop? And should I also send the people who have been helping them cheat a message?

Or should I just let it go, stop participating in this contest and leave the other participants to continue to lose to these cheaters?

If I ran the contest, I would ban them but I don't so...