How we did it at my parents house:
Our house is built about 16 inches off the ground on little cement pylon things, as many houses in California are.
We (or, I, or my dad if I wasn't home) could crawl under there w/ a big giant sprayer thing full of malathion. (that always sucked, there was lots of dirt and lots ofd spiders and typically large numbers of ants.
Anyway, we'd leave the whole area under the house damp with malathion, that would usually get them. If there were still more, we'd do it again the next day, until they were dead. Never took more than two or three days, nad they'd be gone for the year.