Actually, instead of that, why not just assess their voting habits and allow posting at a certain threshold? Those users that continually upvote stories and comments that are considered "low quality" are held, be they new/old whatever. Get too low, and you can't vote either.

The only issue is you need to be very specific in how you choose to asses what is "high" vs. what is "low" quality. Too rigid and you basically encourage group-think, too lax; anarchy.

(Note: I would keep "karma" in place since it would help to distract from the "real" karma system.)