Simply put down an ant trap, you can buy them online or in any hardware shop, very cheap , only about £5 for 2. They are little plastic discs which attract any ants, they then feed and take back to the nest a poison which kills the nest
This is the way to do it!
I used to get an invasion of ants every summer. I traced the nest to somewhere under my kitchen door step and left the little bait stations nearby. It worked like a charm. The invasion has never returned.
I felt bad killing them as they are only looking for food. But their place is not in my kitchen.
If you see an odd ant its best to kill it and wipe down the surface. Otherwise it will go back to the nest and communicate where it found food! Then others will quickly follow.