When I had this problem, I found an aerosol ant killer that contained mint oil which worked really well (I think it was made by victor pest which somehow turned into Safer Brand - either way, I bought it online). I also watched to see where they were coming in. They were sneaking in where the floor met the wall, under the moulding and by the kitchen cabinets. I sealed those spots with a bead of clear silicone caulk. My ant problem hasn't been too bad since then; for the few that ventured in this spring, I put down some mint oil mixed with water and let it dry. I think I read that one of the things the mint oil does is disrupt the scent trails the ants leave, but don't quote me on that.
I have also heard of using diatomaceous earth as a barrier but I wouldn't use it inside. My local pool store won't sell it anymore (it was used as a filter medium) because of possible health risks caused by inhaling the airborne dust.