You can flood the coop area to chase them out.  I then treat my entire yard (except the coop area, or anyplace the chickens eat or roll in the dirt. ) with a professional fire ant killer Talstar thats non systemic.  (It won't go up into the grass my free range birds graze on).  You can get Talstar at a chain called "U killem pest control".  I also run a border around the coop of the killer. It needs to be applied per directions and watered in good several times  before I let my girls back into that part of the yard, I wait a few 2-3 weeks even.  I go to the extent of running a temp. fence to section off my yard to keep them out and still let them free range during the day.  I also don't feed them people food in the coop anylonger.  The feed doesn't seem to attract them as much as food.   It's a battle, Talstar lasts about 9 months before re-treatment is needed here in Florida.  I'm interisted in what other folks do to deter them also.  Ohh, in the coop, I use hay for bedding, and I put de under the bedding also indoors.