No there's no way to 'force' a user to take action, even if there is an answer.
Accepting an answer doesn't 'close' the question, it's just an indicator that the OP found that particular response to be helpful.
Please don't flag these questions just on the basis that there isn't an accepted answer, there's no need (they're fine as they are from that point of view).
What some people around the network recommend you do is leave a comment for the person who solved the issue, and ask them to put it in as an answer. If you don't hear anything back, just answer it yourself.
No need to feel bad about doing that, the goal of the site is to produce a good repository of information - for the most part it doesn't really matter how that information gets to be in the right place. If you feel bad about it you can always mark the answer as community wiki, so you don't receive reputation for it. Although that isn't necessarily the best use of that feature.
See also Why do a lot of people seem to "answer" a question in the comments section?