Get a shovel full of fireants from a different mound and dump it on the first.
This right here works and is very cheap. From my experience, find the mounds that are the farthest apart. One queen can have multiple mounds.
The "worker ants" will go to war and fight to the bitter end, basically committing genocide. I didn't believe it at first, but it works. You can watch them killing each other above ground. Cool stuff.
