As you have observed, they do harm - they steal honey. According to our local ag inspector in San Diego, the small ants can be the worst. There are lots of threads on this forum on how to deal with ants. Here are some starting points:
http://forum.honeyflow.com/t/do-ants-hurt-bees/6004?u=dawn_sd
http://forum.honeyflow.com/t/does-anyone-have-experience-with-ants-attacking-their-bees/3608?u=dawn_sd
http://forum.honeyflow.com/t/getting-rid-of-ants/5740?u=dawn_sd
http://forum.honeyflow.com/t/home-made-pest-deterrents-and-termination/1962/5?u=dawn_sd
Hope you find something useful! I have seen ants destroy a friend’s bee colony, so don’t be complacent! 