This may be because my dad has a little redneck in him, or because I grew up in Norco, buuut.... anyone ever tried gasoline?
When my sister was two or so, she sat on a fire ant hill in the front yard... needless to say, mom was freaked out and dad was angry. But I remember whenever we'd get an ant hill, he'd rake everything (weeds, twigs...and thing too flammable) away from it, pour a small amount of gas directly on the hill, wait maybe 30 seconds, pour a little more on...and light that sucker up.
The trick (or so he says) is to keep the fire going for a few minutes, by adding little bits of gas if you have to, so that it gets really hot deep down into the ant hill. This successfully killed them off for us.
Obviously, if anyone is going to try this, keep a bucket of dirt handy to dump on the fire if it gets out of control...and also, a hose. Keep all the family pyros away, and don't do it if there's a chance that anything near by will catch.