Today its possible that guy whose only activity on forum was asking for free coins 60 times (airdrop) or taking part in few bounties is writing one good post (or get 10 merits in other way as you all know its unfortunately possible) and he is automatically given Member rank. Does such guy really deserve Member rank?
Even in signature bounty campaigns posts from bounty threads are not counted so why we should count it while calculating important Activity parameter.
Summing up: my idea is to cut off the posts written in bounties section from the number of posts used to calculate activity parameter.
Im not sure how difficult it could be from the developer side, but if there are future, which divides all written post into 14-days periods and then calculated only not empty periods probably there would be also possibility to cut off post from bounty board while calculation activity.
I don't think activity really matters anymore. The landscape here has changed over the years and that's why a new metric such as Merit was introduced at a time when more was needed. Other than it's a historical/traditional aspect of the forum ranks don't matter much either. Today merit is the gauge of an account that is more than likely to provide you with information of interest or usefulness. This will be more accurate as time goes on and we've gotten further removed from the air dropped merit distribution.
Your changes are not very difficult for the Admins to implement as it has been done in other sections, but I do not see it as an effective way to keep these members low on activity. They will just spam some other board if they actually need the activity, which is the easiest part of the equation now.
A more effective idea to remove bounties from spoiling the forum would be to remove the ability to merit in bounty threads or boards.