I have tried a lot of ways to kill ants in my backyard and the most effective way was to use Raid Ant Killer Liquid. This liquid can be bought at most departmental stores, they are not cheap but those one small bottle will last you for a very long time. You only need a small amount of the liquid where you see the ants moving around. This liquid poison contains the main ingredient called Borax and it is mixed with some sweet stuff to attract the ants into eating it.
The Soldier Ants will taste a bit of the liquid poison and then they will carry some back into their ant hills where the queen ants will be waiting for its food. Once the Queen ant eat the liquid poison , it will die and it will not be able to reproduce anymore ants. The remaining ants which have eaten the liquid poison will eventually die too and that will be the end of the ant hill colony. It may take a few days of putting out those liquid poison to completely wipe out the ant colony.