Step 1: Obvious and Not So Obvious
My first way can be somewhat expensive, but very effective GASOLINE. If you pour some gasoline on ant hills they will quickly die. But money doesn't grow on trees (maybe someday) so the next few steps are much more cost efficient.
Do NOT attempt gasoline method in dry conditions. I am not responsible for what you do.
Step 2: Boiling Water
Step 3: Alternate Methods
But, we are trying to find easy and cost-efficient methods. Please be smart and realize there are a lot more ideas out there. Hopefully this just jogged your memory to think up some real creative idea. Be safe.