Did you know that ants are generally not a garden pest? Nope, they are ok to have in your garden.
When would an ant be a pest? When it is playing bodyguard to an aphid (tiny plant sucking insects and can wreak havoc on a garden). While aphids are happily sucking away the life force of your treasured plants, they are also creating a sweet honeydew that ants just love! So much so, some ants are willing to fight off insects that would prey upon aphids.
If you think they are taking over, encourage their natural predators to help keep their numbers in check. What eats ants? Insectivorous birds (birds that eat insects).