To kill ants without any damage to the environment, I would try getting a tank of carbon dioxide (CO2). Attach a length of pipe so that the CO2 comes out the end. With the CO2 flowing, press the pipe against an opening from the nest. The flowing CO2 will blow soil aside, effectively drilling a hole into the ground. The CO2 will kill with two effects: 1st, lack of oxygen displaced by the CO2, 2nd, cold will freeze the ants or at least slow them down. I have not had an opportunity to try this, but it makes sense to me. Good Luck!