If you are sure you have ants (not termites) then you should try one of the effective liquid ant bait products. It is a mixture of boric acid (borax) and a sugar syrup. The worker ants bring it back to feed the queen and the larva. Takes about a week to kill off the colony.
Both Terro and Revenge do the job.
http://www.terro.com/
http://www.get-revenge.us/antkiller.html