There are no automated penalties beyond the reversal of the votes, but moderators can take action.
For a one-time serial-voting episode that gets reversed, most of the time (in my experience working with other moderators) there won't be additional consequences. If it happens more than once or is egregious (lots more votes), moderators will probably at least warn the user and might suspend. I've suspended people for a first offense when it involved lots of votes, but usually, if it's just a few votes and the user doesn't do it again, I figure either the reversal got the message across or he got it out of his system.
Once we know that a user has done this, at least some mods try to keep an eye on the user for a bit in case it repeats. Don't expect that kind of monitoring on SO, where the mods are already way too busy, but on smaller sites where this doesn't happen a lot, moderators are more likely to be able to pay attention.