You have to be careful of where you put the poison but most exterminators will tell you that poison is the most affective way to get rid of rats. They carry it back to the colony and it will kill several of them.
Poison is the best way if possible. I had rats over the winter but with two weeks of poisoning it took care of them. The first 4 days they were carrying off 5 to 7 blocks each night. They stopped taking them and I found some dead. I now keep blocks and change kinds every few weeks to make sure...