A couple of times it happened to me that I bountied a question. If nobody has answered, the day before the expire date of the bounty some guy answers with some stupid answer, just to take the bounty prize. How can I avoid this?
If the answer is of very low quality, flag it as such. Other users will review this flag, and if necessary, the moderators will intervene.
If you see a pattern of this behaviour in some users, or if you suspect foul play, please flag this as "in need of moderator attention" and explain the situation.