sprinkle cinnamon over their path. The cinnamon kills the scent trail, which confuses the ants and they disperse for parts unknown.
It is messy, but it works. I guess you could find some cinnamon scented oil and do the same thing, and it wouldn't be as messy.
We sprinkle some cinnamon at all the doors to the house periodically, because that seems to be their easiest way in the house. Probably because we drop so many crumbs if we're eating when we go out the door!