While "it is likely impossible that [you] have time to look at each question on the ongoing contest," I recommend picking the Questions you choose to invest in with consideration of the following:
(1) Assigned exercises (and contest problems in particular) are likely to be mathematically sound and interesting (they are intended to be challenging, at least to the extent of reinforcing course lessons). People who post these assignments with the intention of cheating will nearly always quote the problem statement carefully without surrounding context. Your linked (self-deleted) Question is a good illustration. The OP showed no "digestion" of the problem's meaning or difficulty.
(2) You may have unintentionally frustrated the OP's hopes of having a solution that could be cut-and-pasted to turn in as if it were their own work. For one thing you changed notation ("my notation is not as you stated, but this is not important").
(3) The website displays a message "New contributor" just below the OP's name and reputation stats. The black "hand" icon is meant as a warning, not only to be nice but also to be wary of someone unaware of (or uncommitted to) Math.SE's goals of collecting and curating excellent content.
(4) Finally one might reflect on whether your efforts benefitted your own learning, regardless of bad faith by a rogue user or the failure to retain good content for the Community. [Often the Questions that I find most rewarding, from the perspective of my own learning, are drawn from "real world" applications and have a very different gestalt from the "pass through" assignments that cheaters will post.] So if you got an intellectual reward from tackling that challenge, I'd say you made a good choice.