Ants, if you can find their hill, pour boiling water on it. If it doesn't kill the queen the first time, try it again.
If you can't find the hill, sprinkle dry clothes detergent or dishwashing detergent where they can get to it. There's something about the detergent that attracts them and they carry it back to their hill. What they don't understand - it poisons them.
Ants like dry, light soils, so by applying a regular drenching of water via a sprinkler or rose watering can will make them pack up and move.
The herb 'Tansy' has a very strong smell that discourages ants from setting up home. So plant some Tansy in your garden.