Please, for environmental sakes do NOT use any oil based chemicals on an anthill. Boil a pot of water, and dump it on top of the anthill, it will soak thru and broil those little bastards with extreme prejudice. If it doesn't get rid of all of them, do it at least one or two more times.
If they prove to be resilient, purchase some decent ant killer. But please do not dump gas/petrol or other oil-type products into the ground as it will also kill any plantlife as it seeps into the groundwater.
If you're hell-bent on setting fire to them, use any form of alcohol.