Make sure you read all the posts because there are lots of suggestions. Off the top of my head, I would try what this poster stated:
Borax and suger…i was usinf 2 cup suger 2 cup borax 2gallons water. water must be hot stir until suger dissolves. soak mound around the nest just enough to wet mound. leave for 24 hours and your promblem should clear in that nest. solution must be dissolved. outdoor only because it could be sticky.
If you can't find the mound, we have taken a medicine cap and filled it with the solution and watched them drink up. Just put it in their path to find it. If you can't find Borax we actually used Boric Acid. I think it took a few days but never came back.