It surprises me that only 15 people have reported more than one post in 3 months. That doesn't show much community involvement (although I don't know the numbers for other boards).
Most people probably don't want to waste their time with it. It takes time to report things and there's no real benefit for people other than helping the community out.
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.
As I've mentioned before it would just be exploited by people if you could earn merits this way. I wouldn't be against rewarding people for reporting in some other capacity or with additional perks but merit shouldn't be given out for it as it's just asking to be gamed.