There has been a lot of heated debate on this issue lately and as a user, I decided to propose a definite policy on such matters:
The following is my proposed policy
1) When a problem from an ongoing contest is posted and someone has pointed us to the source of problems,
we should ask ourselves
a)Is the competition a part of the selection procedure for a particular country's team for the International Mathematical Olympiad?
If (a) is true, we should ask ourselves this
(i)Is the question a problem from a particular contest in the past or a well-known theorem
If so, I propose that the question be kept open.(Olympiad organizers are not supposed to propose well-known problems)[The onus of proof that the question has appeared elsewhere earlier should be on the poster.And we have enough competent people to judge if the problem is a well-known theorem or problem or not]
(ii)The problem is not well-known one at all.
The question should be promptly closed and comments disabled on it till the contest is over
b) If the competition is not part of the selection for the International Mathematical Olympiad, moderators should put up a sign saying: "This is part of the (name of the contest) competition.Please use your judgement while posting answers" and the moderators should not try to close the contest.We cannot be responsible for every little contest.
Please use upvotes to vote in favour and downvotes to vote against the proposal.
Edit:This is going to be debate;let us take all viewpoints into consideration as no single viewpoint will likely to be overwhelmingly useful for the site.My attempt is merely an imperfect, but perhaps a necessary one.