Wash the poopy butts.
Seriously, there are many causes of poopy butts, from simply too much "wet" in the diet to a condition called vent gleet. Sometimes worms can cause it. Sometimes coccidiosis can cause it. Sometimes it's a simple matter of fat accumulation below the vent, causing the poop to cling rather than shoot beyond the butt.
You can try fermenting your feed and see if it helps. You could try ACV in the water, and that could help. But if anyone has a surefire cure for ordinary poopy butt, I'd sure like to hear it.