Some days ago, I answered a question. This question had an open bounty, so if the user who asked it marks it as approved should "pay" 50 points. I believe (well, I'm sure) the question is answered and it should be marked as valid, but the user don't do it and the bounty will finish in 2 days. I've asked the user for more info about why he don't mark the question, but he is not responding.
What can I do? Is there any mechanism in Stack Overflow to report this kind of unfair behaviours?