I started to notice a lot of new users post their first question and, then, disappear. They don't close their topics (by accepting an answer).
It is created lots of unanswered questions (although there is often a good answer proposed). It is a waste of time for users who are looking for (true) unanswered question.
Is there some mechanisms to close these questions?
It's not a problem. If there would be upvoted answers, the question would not be unanswered and would go away from the "unanswered" list.
No need to close, what would actually prevent further, possibly even better, answers.
See on the main meta site: Would it be possible to have a “community accepted” feature?
You can show your acceptance of an answer by upvoting it, even if the OP didn't show his preference.
