We have the same problem, so my Mom recommended Terro Ant Killer. It's borax based and takes them out, that's for sure. I like ants, I just don't want them in my space. I don't invade THEIR space (I container garden *lol*) So I want the same space respect :)
we tried grits on the outside, was something I found on a web search it worked for a while but they are back now and they brought reinforcements...i'm leery about using any kinda of chemical especially in the house because my kids have a tendency to get into EVERYTHING, lol
I've read that ants won't cross cinnamon, and if you sprinkle it across their path, they have to find a new way in.
I may have to buy a grip load of cinnamon then, and figure out how to pipe it in my walls... in the end i may have to tent my house...:(, apparently for the 4 years I as not living here they just built a mount under the house, a very very very large mound...