If something is on-topic for the forum, but say, in the wrong category, then admins or mods with sufficient trust level can move the post to the correct category. If a reply is off-topic for the current thread, it too (and any replies) can be moved to a new (or existing) thread in the correct category.
But if something is off-topic entirely, or is a personal attack, etc., those can and should be flagged.
As for those selling stuff that's not related to keto, I'd flag those for deletion. Selling stuff related to keto can be moved to the SPAM category instead.