What if mods reward members a merit for an accurate report of spam/useless posts topic? This way more users will have more initiative to report those kinds of threads/posts whenever they see one. This will also help the forum as a whole if there are a lot of members who will do so, but I guess it would add a huge workload since the moderators will get more report than it is now. This is just my opinion tho.
Merit system should be used to create new and more useful posts while decreasing the number of spam posts.
By adding report rewards, you're only acting upon the latter. Plus, you'll probably see a lot of spam reports if users are scrambling to get merit now through strange ways, then surely they won't care about doing bad reports as long as they can get some good reports through (which result in merit)?