I use BATS Bayer Advanced Tree & Shrub which is a systemic insecticide. It is used in a reduced dose and works well for root mealies and other pesty bugs. I should think it would work with ants also. If you should chose to use this, the measurement is 1 or 2 Tbsp per gallon of water. Watered in when the plant is dry so it absorbs well. Hillbilly_Gran uses a granular form of imidicloporid which should work the same.
We have fireants here and if I have plants they want to take up residence in, I sprinkle a tad of ant poison on the ground under the pot to discourage them.
I don't know if this is a viable long term solution for you, but can you set them up off the ground? I put the legs of the plant stands in deep trays and fill them with water. Ants won't be able to get to the container, then. Of course, if the container is too big, this isn't an option. Good luck!