In most of the other forums I'm on, there is a 15 post minimum before PM's, but it is 15 USEFULL posts, if its not a relevant or informing post, it doesnt count. Surely that would be the way to go here?
Also, regarding closing threads after a certain length of innactivity, why not close them, then archive them in subcategories within said category e.g. 'Black Hat topics from 2010' or whatever then people can go off and do their search but they will be able to read but not comment. If they have a worthwhile and usefull comment then i'm sure people wouldnt mind them bringing it up in a new thread?
also, once somebody bumps an old topic, everybody else tends to not notice its a bumped topic and more comments start coming in. I for one am guilty of just checking the date of the last post rather than the first post, I shall be a bit more wary of this in future!