I use to have a really bad ant problem at my old place. I tried a lot of different products. The only one that actually worked was Terro bait:

http://www.terro.com/products.php?product=liquid_ant_bait

I actually can't believe how well it worked. The link is to their indoor bait which I used. If their outdoor bait works half as well as their indoor bait, it will solve your problems.