Is Lighthouse from Dominion: Seaside overpowered?
If there are tons of attacks, sure, it's pretty useful. But setting aside the defense (which often isn't relevant), the coin it provides isn't really terribly powerful, so no, it's not at all overpowered. 

It's basically a copper that gives you one more coin the next turn. In order to get that extra coin you had to add a copper to your deck - the turn you play Lighthouse one of your five cards only gets you one coin. A silver generally does more for you.

So you don't really have to do anything to keep people from abusing it. Just buy bigger better cards. You can do a lot better than +1 coin.