Review Suggested Edit

You can't approve or reject suggested edits because you haven't yet earned the Edit Posts ability.

^
That can be used.
You can try to contact the person who posted the challenge and have them mark the answer as "React -> Invalid". Or otherwise it seems that anyone can mark an answer as invalid? I think this was the original purpose of that feature for this site.
This is otherwise non-disruptive and gives the one who posted the answer a chance to fix it. Doesn't seem that it requires moderator involvement either(?).

^
That can be used.
You can try to contact the person who posted the challenge and have them mark the answer as "React -> Invalid". Or otherwise it seems that anyone can mark an answer as invalid? I think this was the original purpose of that feature for this site. The help text for the reaction also suggests this usage.
> The proposed post/solution does not comply with the rules of this site/challenge.
This is otherwise non-disruptive and gives the one who posted the answer a chance to fix it. Doesn't seem that it requires moderator involvement either(?).