Ya know... I think that the best way of trying to do what you said is on a post-count basis.
Think about it. After making 150 posts, you must have learned something. Plus... once your past the 150, you really don't even thing about spamming. If you want to say something useful, then you post it. Otherwise, you just don't waste your time. So... Brandon, not 100% sure of how vB handels permissions, but i'd think that the people that have earned 'custom user titles' most likely would be ok people to be able to reply. But... still have plenty of mods so that if a user just posts the same thing over, it can be removed or if a discuession starts. Also, be sure there is a way that you could prevent a over-150 user from posting if they abused their 'power'.
I understand that just beacuse you have 150 really doesn't mean too much. Also, for past users, blocking count from the Chit-Chat forum doesn't really do much, but for feature users it will make sure that they will only be allowed to post after they've hade 150 good 'first' posts.
That's just my though. The only down-side is that it might make new users want to spam more so that they can to post in the forum. BUT... word to all of you newbabies: It really isn't that hard to get 150. You'll be there in no time.
Just don't spam to get there or it will only take longer. 
Oh.... Another idea. I know it would be a little anoying, but what about some sort of reminder whenever posting a reply in the Q&A forum? Like a little JS pop-up that whenever you click the 'submit' button remindes you and asks if you are sure.
Ex: "Remember, you are posting in the Q&A forum. Is this a good post?" - Yes/No
I know it might get anoying, and mabey could be dissabled for the mods of the forum... but would just give a reminder to users to see if their reply is really neccessary. Be sure that the pop-up only occures on replys though.
Have fun Brandon
Jack