There is a fair amount of discussion about contests at the moment, in the context of active problems being posted to the site for answers and needing to be closed.
Bearing in mind that
(a) contest problems are often both challenging and interesting and might well be of interest to users of the site; and
(b) there are so many contests out there that no-one can police them all
(c) the general philosophy of the site "open" rather than "closed" ie good mathematical content is welcome
might it help if there a deliberate part of the site where promoters of contests could post details, and users could locate active contests if they were interested.
I know it is not the main business of the site, but at the moment there are a few ad hoc notices, and that will not work well at any scale. There would have to be some careful thought as to whether this could be done without requiring heavy moderator attention.
It might then be possible to ban users who post advertised questions, for example, as acting contrary to site policy.
There may be other suggestions out there. It seemed to me that reading the discussions about closing questions or soft deleting answers, it might at least be worth thinking if there were other ways of responding which might enhance the utility and mathematical content of the site.