Questions only bubble up with “modified by Community” if they have no answer that's accepted or upvoted. So anyone with at least 15 reputation can stop the question from being bumped by upvoting one of the answers. Of course, upvote only if you're confident that the answer is correct.
Only the asker can mark an answer as accepted. If they haven't accepted an answer, but they've clearly indicated in a comment that they're satisfied with the answer, you may add a comment pointing them to the help center page “What should I do when someone answers my question?” and suggesting that they accept the answer. Do not badger askers into accepting an answer if they haven't indicated that they're satisfied with the answer, it is perfectly valid not to mark any answer as accepted if you find the answers unclear or incomplete.