What can we do about users with a 0% accept rate?

Question

Do we not answer their questions? Is a moderator able to step in and accept a clear, solid answer?

Answer 1

Accepting an answer is not mandatory for a user - so you should not be "boycotting" users who have zero accept rate(yeah, I know that can be hard) - neither should you be badgering the user to accept their answers. Of course, few users aren't aware of the concept of accepted answers, so I generally leave this comment.

You have a bunch of questions that have been answered and has obviously helped you, but you have not accepted them. Please do else people may be not be inclined to help you.

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Coming to your other question, no, Moderators cannot force accept answers on any arbitrary question, barring their own.