Boric acid is toxic to ants. When the insect ingests a small quantity of boric acid, it doesn't kill it right away – slowly but surely, the ant will be poisoned. The way boric acid-based bait works is simple: It attracts foraging ants, which swallow it and hold it in their stomach, without dying.
You can get Boric acid on Ebay.
Then you need to do this..
Mix 1/2 C sugar, 1 1/2 Tbsp Borax, and 1.5 C warm water. Soak some cotton balls in the mixture, and put them out near the mess of ants. The sugar attracts the ants, and they'll take the Borax with it back to their home. Eating the Borax kills the ants.
I have only ever had ants once and this is what I did, I aslo gave some to my hairdresser who had them in a marching line from her kitchen to the back door, it was like something out of a cartoon.
She used this and they disappeared over a few days.