Just make sure they didn't move out into a nearby mound or tree. When I lived in Houston a colony moved around the back yard a bit as I tried various ways of killing the queen. At the time (1997) I could buy dangerous chemicals from a nearby store. Self-Chem they were called (maybe they're still around, who knows) but it was a chain (!!) that sold industrial/commercial chemicals in small quantities for household use.
They sold me something for fire ants that they said (paraphrasing across nearly 20 years) "You spray it on the mound. When it dries, it crystalizes, and the crystals stick to the ant hairs and get slowly worked upwards and into the body to be absorbed. Then the ant's dead in 2 hours. But not before he's had a chance to rub up against all the other ants in the colony. Eventually the toxin makes its way via the ant labor system to the queen and kills the mound. It takes like 2 weeks or more, if memory serves.
Nasty stuff.
I wonder if that's even legal any more.
Oh, and the sign on the store: "Self-Chem:Your Neighborhood Chemical Store"