I suggest the following procedure for eliminating ant mounds in lawns. First, rake the ant mound down level by spreading the dirt to the surrounding area. Wait several days to see if the ants rebuild the mound. If they do, you can chose to use an insecticide to reduce the population rather than fight the mound with repeated raking. After the mound has been raked level, sprinkle a small amount of diazinon granules or Sevin dust onto the area.
Source: Ant Control in Lawns - Iowa State University
Many pesticides are harmful to the environment as well as the pests you are trying to eradicate. If you use boiling water for getting rid of ant hills it is completely safe and quite effective.