My bounty was Pyrats but nothing I do would count them, including killing them all 3 HP myself.
But just clicking it worked:
Turned it in, told me I completed it and got 1,000 gralats but did not receive the gralats and the box is still following me.
Pressed "A" while facing the dungeon master and it took the box and said the bounty expired and did not give me a reward.