There are some stump-rotting products on the market, but they don't appreciably accelerate the process. The quickest way to get rid of stumps is to have them ground down with a stump grinder. You can rent the equipment yourself, but it's best to hire a reputable professional tree-care firm to remove the stumps. Their employees will have the know-how to do the job right. Make sure they grind the stumps down at least 6 inches below grade so you can successfully grow grass over the spot. An alternative that is cheaper and will accelerate rotting somewhat is to cover the stumps with a dark tarp and a pile of mulch or soil. Keep it moist. This will be unsightly for a while but will do the job eventually. You could plant some morning glories in the mulch to hide the piles during the summer.