This is my humble suggestion:
Have a MAXIMUM number of activities or posts for junior and Member ranked profiles in Bitcointalk per week or month.
I see people with 200+ activities and only one merit that they probably bought/scammed someone to get. THESE accounts are the real shitposters.
This is an interesting idea. I don't think it should be restricted to only the lower accounts, but should perhaps be more focused on limiting the creation of new threads as opposed to responding to topics. There are plenty of higher level accounts spamming the forum with new threads in an attempt to garner attention and earn merit also.
I don't think this will matter to low ranking accounts: a spammer can just open 10 more accounts if his account can't post anymore.
For high ranking accounts on the other hand it could work, I don't think a Legendary spammer would want to continue with a new Newbie account. But I don't think theymos is very likely to restrict freedoms like this, so I don't expect this to happen.