After the business "next door" cleared out the desert lot, an entire ant colony moved underneath my house and only my house. Every summer/fall when it rains, I have ants. It's really depressing because they will create colonies where I know I have buried pets. Anyways off the depressing topic and onto solutions..
Diatomaceous earth is a good way to ward off ants, put a line of it where you don't want them going. Sprinkle bits in their lines if you see them. If they walk through it, it will cut up their exoskeleton and basically dehydrate them.
Borax mixed with any number of "foods" for ants will kill them off. If you want instant deaths, use more borax. If you want them to take the borax back to the colony, use less. As fearaffect said, 5% or so would be a good amount.
Cinnamon, cucumber peels, practically anything with a very strong smell tends to keep ants away. It does not kill them.
Terro ant baits are premade ant killers that use borax. It's a little expensive but it comes with it's own "house" to put the bait in.
Corn meal is another option, but it doesn't work with all species of ants. Generally speaking it's not considered humane because once an ant ingests it, they tend to expand to the point they cannot expand any further without, well, dying. I use corn meal as a last ditch effort.
If I had photos of the ants after it rained, you would understand why I have had to learn so much about them...