I have used this formula a few times with good results:
Borax will kill odorous house ants, and powdered sugar will attract them. Make a homemade bait by thoroughly mixing one part borax with 3 parts powdered sugar. Fill tiny containers (such as bottle caps) with this homemade bait and place them as close to the place where you suspect ants are entering your house. If you see trails of ants, place small containers of the mixture directly in their path. This prevents most of them from traveling all around your house if they have easy access to this sugary treat.
(Have pets or kids who might get into this powdery mixture? Try this instead: mix one cup warm water with ½ cup sugar and 3 tablespoons borax. Soak it up with cotton balls and place them in shallow dishes near ant trails.)
Resist the urge to kill all the ants you see. They will carry the bait back to the nest, unable to differentiate between the borax and sugar, and the borax particles will eventually kill the entire colony. The more ants that are allowed to find the bait and carry it back, the faster the colony will die off.