We constantly battle fire ants here. When we find an ant hill, no matter how small, it must be eradicated immediately. I'll tell you what we did (before we had chickens) that knocks them right down, but it isn't organic and you might not want to do it. Take a few tablespoons of gasoline and dribble it over the mound. It doesn't take much. The gas fumes are heavier than air and the fumes sink down into the tunnels. Within a few hours, the mound is dead.
We don't have problems with ants since we got chickens here. Nowadays, if I find a mound the chickens haven't spotted yet, I call the chickens over and they devour the ants and then dust bath in the dirt mound that is left.
So yeah, gasoline and chickens are my two top defenses against ant intrusion.