I take down the hills and drown with Raid ant killer.
I recently bought Sevin and it has been great to get rid of every kind of ant in my yard. Sprinkle a cup on mound, then a little water. They are gone in a couple of hours and I haven't seen them re appear. It's been 3 weeks now.
Before that I would poke a hole in the mound with a stick and pour boiling water in it. It worked but is dangerous on rough terrain.
Amdro Ant Bait Granules for fire ants works quick entire colony gone 2-24hrs
Depending on your problem - where the ants are - there are a variety of methods. If you have a hill or two, the above answers are just right, Seven or Amdro or the like will get rid of colonies pretty fast.
If they're in your house, then you will have to do some investigating and find out where they are coming in. (sometimes they'll travel several yards from the mound to get in the house.) To fix that, treat the mound, seal the hole (or holes) that you can see and then it would be a good idea to do a border around your foundation of one of the products (they all work about the same) that work continuously for 6 months or so. I treat ants I find IN the house with diatomaceous earth. Just dust them and the cracks you find them coming out of. Vacuum when they're gone.
If the ants are in the flower beds and seem to be all over the yard - then buy a spreader and use whichever product you prefer for the 6-month treatment.
Fire ants will also colonize vertically. I've found them in PVC posts and poles and in rotting trees. With all the water spring has brought, you're bound to find them if you live in the south.