something should be done like restricting newbies and jnr members from commenting on thread but can access thread to get vital informations. so the spam level can be reduce to its minimum.
Restricting them completely from posting won't be a necessity if their signature spaces are restricted. That would reduce the spam itself without even having to restrict them from posting, because most of these spammers are bounty participants and their primary purpose is to reach the minimum/maximum post thresholds that their campaigns offer so that they don't stay behind in getting payments. And maybe, there are some low rank members who can post constructive things and deserve attention from higher rank members. Such members won't be able to show themselves if they won't be able to post just because of some other users who used to post crap all the time.