Anything sticky. First I used hairspray that is strong, usually doesn't work until a lot has been used. Then I tried glue, didn't bother them, they just probably later die from tasting the glue if they did.
Today, however, I found a thing of Gorilla Glue, one of the strongest glues in the world that you can get. And boy, I must say, only a itty-bitty bit of that glue on an Ant, and they'll stop moving after a minute and be stuck, literally. If you apply a bit, they literally become encased in the glue, and since the glue is yellowish and see-through, it can become a fossil sort of look. I glued a rollie-pollie bug today in the glue, and it was there until I later washed it away with water encased in the see-through Gorilla Glue.
I know what I'm going to do tomorrow when I'm bored after school.
(I also like to use a lighter and burn the little buggers. Am I mean? Probably, but I find burning a fire ant oddly ironic, and after watching Indiana Jones, I no longer have a sense of empathy for them when I see them, especially the ant hills not too far away from here. One day I'm going to get a bit thing of glue, drop it down the ant hole at the top of the hill, run away, come back five minutes later, and see all the fossils I'll create!)