I believe if you pour boiling hot water into the ant hills, it should eliminate the ants. White vinegar can be used to stop them from following their trails back to the ant hill, but you have to be careful with that as it can also kill plants (but it's great for killing weeds!).
If you'd rather a spray, I use an insecticide that uses natural oils to kill ants and other small bugs. It has rosemary oil and peppermint oil as its main ingredients, but it includes wintergreen oil and others as well. The smell is incredibly strong but the spray is effective and safe around children and pets.
PracticalMom, I'd heard of this and totally forgotten. I've lived in areas of horrid fire ants and remembered, but occasionally it seems to make the rest of them mad, so you've got to have plenty of it if it's a large mound.. Thanks for the reminder! Gusser, that's a new one on me! Don't know if I can stand the idea of exploding ants or not, but who knows? May need to try it someday. (My grandsons would like it!