Boric acid is the only thing that’s worked for me. (Caulking, chalk lines, talc, cornmeal just seem to encourage them to try harder.) It’s supposed to be as safe as table salt, although you have to keep it away from pets and small children. It’s a desiccant. Terro ant traps have a boric acid and sugar syrup mixture in them — they work really well. Sometimes you’ll get a big carpenter ant guarding the trap, but it dies and then the red ants fill it up. If you know where the nests are, you could just mix up a boric acid and syrup solution and squirt it down the holes. They have boric acid at hardware stores — it’s used to get rid of cockroaches, too. Smells like a thrift store.