I don't think there needs to be any action taken against anyone voting or anyone meriting for votes, I don't think that's something that even needs any in depth discussion, I just think in order for the event not to be ruined, and made as fair as possible, possibly the admins could limit merit in the thread in future events. I'm not sure that they would do that, I just see that as a solution.
When you say limit, do you think the forum software allows for modifications where theymos could "blacklist" a specific thread, so to speak, and make it impossible for anyone to send merits for posts in that blacklisted thread? If that is possible and theymos cared about doing that because he feels this whole thing is a problem, that could be done in the future. Personally, I don't think he cares.
It's even easier to not have the admins involved at all and make the rule sound something like this:
Any member voted in isn't allowed to merit the person who voted for them. For each merit sent, they will be deducted the corresponding number of points when announcing the winners. So if you feel like meriting your voters, sure go ahead. But 10 merits sent to them results in 10 points being deducted from you in the total count.
But if you wanted to cheat, you could easily bypass that as well. You merit some other post of your voter, just not the one where they voted for you. Unfortunately, there isn't a perfect system.