Seems that only admins (or forum moderators) are allowed to set some topic as answered which is logical (to avoid malicious people added questions and faking some answer from other account to raise their reputation)

Microsoft forums though have a "Propose as answer" there (this is available for author of some reply too to suggest this is an answer and not some related question/clarification of question) that the admin/moderator can see and accept

this is also related to
http://www.mono-software.com/Mono/Pages/Discussion/dtopic/8GL0hGkExU6ZLqEEAP189w/Can-39-t-unmark-an-answer/?dmsgId=A77i5cgBe0SAa6EOARe7KQ#PostA77i5cgBe0SAa6EOARe7KQ
obviously if that item is addressed (be able to unmark answers), I'd also suggest that users should be able to "Flag as not an answer" items marked as answer (and notify the board owner etc. same as when using "Propose as answer" suggested above)