Soapy water will kill them, but I don't think it will treat the area and deter them from coming back. I left a water hose running in a flower bed outside our back door a few weeks ago and it flooded a fire ant nest. My wife stepped out the back door and started hollering. The ants had swarmed up onto the back steps, up the side of the house, and were coming in the back door. A bazillion fire ants everywhere. I took some vinegar out there and poured it on the steps, but they happily swam around in it and seemed to enjoy it. I grabbed a bucket of soapy water and poured it on the steps and they immediately curled up and died. I threw soapy water on all the walls and poured it on the mound. They were gone in no time.
If you just want to spot treat you could try some in a spray bottle, but to get rid of them you need to find where they're coming from and treat the colony.