If you can follow a few ants back to the nest, and the nest is accessible (like the hills in one guy's backyar, or probably in the cracks between boards on another's deck), then the single most effective, non-dangerous to humans, and environmentally friendly weapon is boiling water. Crack open the hill and pour a few large pots of boiling water on them. They're done, instantly.

 

If boiling water doesnt get them I usually use Boric acid. If you mix about 5% with something they like to eat, grease or honey for the common household type, they will chow down and feed it to the whole colony. In about 4 days they get a tummy ache and die. Google boric acid and many garden forums will show what to do. Boric acid is available at any home center as roach killer. The fine print states that it also works on ants. Be sure you don't mix your bait too strong because you don't want the workers to die before they feed the colony. Most ants have 2 stomachs, one for the colony and one for personal use. The Hotshot Max Attrax roach powder is 99% boric acid, way too strong. I have had good success mixing with butter, graham cracker crumbs, or honey. The 20 mule team borax soap also works from what I have read. I have had the ants waiting for me to put our their morning snack just like cows at the feeder. Then one morning they are all gone. Also 409 kills the kitchen ants that look like a crawling bread crumb if you want to spray and watch 'em squirm.

I have been told that a clean kitchen prevents them but my ants were so small that one cheerio would feed them for a month, and I have a 2 yr old child, a constant crumb factory.

Hope this helps

 

Terro i the way to go! I've used it successfully over and over again. Just a little dab near their trail, and they'll gather around like it's feeding time. A day or two later, the whole nest is dead.

For a quick kill, try Windex or anything with ammonia in it. Kills them almsot instantly. Just be careful if you have cats, they like to pee where they smell ammonia.

 

I'll recommend the TERRO as well. It's available most anywhere and it's easy to use. It's kind of neat watching all the ants lapping the stuff up knowing that they won't be around anymore.