Amend the forum to hide signatures of any account that hasn't earned at least 1 Merit in the last 2 months.
This will bar them from getting paid to post and not have any negative impact on anyone making even the tiniest contribution to the forum. Nobody loses their rank and even if someone is away from the forum for a while they can quickly get their signature back.
Even when it is not a bad idea, I don't agree with you on the point of merits. Recently I have seen too many farmers giving merits to their accounts, even when the publication with merit was a total crap. In order to make your suggestion even more accurate, I would change the part about merit and encourage signatures managers to check out the merit background of their users, as well as their last 20 publications. This is really easy to detect here on the forum merit-farmers if you take a look of their profiles and check out their merit, for they are usually shameless and give merits from an account to another almost every day.
Nevertheless, I find your suggestion truly useful to the forum, and my little modification is focused on the only part I consider to be improvable.