There is only ONE thing that will get rid of fire ants: a product called Logic or Award. It's a growth inhibitor that when picked up by the ants as food, prevents the ant eggs from developing and hatching. It takes about 4 weeks to reduce a mound. It is NOT a poison and will not kill ants or anything else. Perfectly safe around pets and children.

It's expensive but it works. You just have to keep after the ants continuously so that the eggs never hatch. I used to buy the stuff in 25 pound bags at the farm/feed store.

The problem is that if you have neighbors who don't treat their fire ant problem, their ants will always be burrowing onto your property. They're a pestilence.