Now at the current situation you guys are banning accounts that has posted a lot of useless posts. But you are not looking for solution to this. So what you want Is only constructive posts on forum? You should set exact detailed criterion for posts. Who doesn't follow them should get a warning, and give exact time to stop posting posts that doesn't meet criterion. If the time exceeds to start making normal constructive posts ban for that account should be applied BUT I don't think It should be permanent. Bans for accounts that keeps posting useless posts should be up to 1 month in my opinion. When time expires and he/she starts to post useless posts again, he will instantly get 30 days ban again.

I think that would be a solution? What do you think?