Here is what I do for ants that nest in my pots.
If you're organic, sit the pot on some diatomaceous earth and sprinkle some on the top. The idea is to make sure that foraging ants have to step on it. Will take awhile but if you are diligent about keeping up the DE it will eventually kill them.
If not organic then sprinkle some granular insecticide underneath the pot and some on top of the soil and water. That should take care of them in about a week.
For future reference I mix up about a quarter cup of DE to a half gallon of potting mix. I use the DE laced potting mix on the bottom of whatever pot I'm using, about 2 - 3 inches worth then fill the rest with untreated potting mix. It seems to do a most excellent job out of keeping them from making nests in pots.
