I'm not sure if we need a new policy. Often such questions will fall under our close reason "Requests for analyzing or deciphering a block of data are off-topic here, as the results are rarely useful to anyone else". In those cases, I suggest closing them.
In general, I suggest we discourage posting contest questions here before the contest is over. I don't know whether we need an official policy; the community can communicate its norms by downvoting such questions. People are free to vote on the question to express their view. If you discover that a question is a contest question, I suggest posting a comment mentioning this fact and providing some verifiable evidence (e.g., a link to the original source of the problem), and downvoting the question if you consider that appropriate.
Finally, remember that posters are expected to credit their sources. Plagiarism is not cool. If you spot an instance of plagiarism, flag it for moderator attention, or follow standard guidelines for handling plagiarism. If you spot an instance where the user posts a question and fails to cite sources appropriately, post a comment to let them know of our policy or flag it for moderator attention.
If we needed a new policy, Math.SE's policy looks like a well-thought-out policy we could consider, but if this is rare, perhaps for now we can get by with our existing close rates plus a little bit of education and voting.