Bayer Quantum. Find a few of the ants paths and place a few small drops of this gel nearby. The ants will soon gather around to collect the gel and take it back to the nest, where it kills the colony.
Its quite expensive, at about 300THB for a small tube, but you only need a few drops to do the business.
IME - it eradicates them for a couple of months or more until you need to repeat the dose.
Sold in Tops, Big C, Lotuss etc.