There are automated and manual measures that moderators and admins periodically check.

Exactly what they are, how they work and what they do, I won't go into... For the simple reason that it sets bounds for people wanting who want to cheat the system. If people are going to cheat, we want to trip them up at the first hurdle.

But in answer to your question, yes. Any person found trying to cheat at the very least has their "work" undone and we have a little chat with them. The more persistent cheaters are hanged, drawn and quartered.

Occasionally there are more subtle issues like people knowing other users in real life and that can have a negative effect on their voting patterns. That's what http://meta.askubuntu.com/questions/5267/guidelines-when-voting-for-friends is aimed at. Even if that post doesn't apply to whatever you're really asking about, it gives a solid background on *why we act* when we detect abnormal voting.