I know it's late in the season, but I was just reminded of a great way to deal with ants. A few years ago we'd bought a home that had been sitting empty awhile. There were numerous ant hills on the property. I tried various poisons with mixed results.
One day, gardening, I'd had enough. We'd been recently talking about the advantages of pouring boiling water down drains, and somehow I got the idea to try it on ants.
I boiled up a couple of big pots of water and poured them both, one after the other, directly on the anthill. Well it worked like a charm and in an afternoon I was able to just about eradicate all the ants on the property.
Ok, it's not free, but so close to free, and completely organic, and it works! So do the world a favor and give it a try.