This suggestion from o_e_l_e_o
A suggestion I did see in another thread, which I would prefer over this one but has a similar outcome, is charging users 1 earned merit per activity period (2 weeks) to display a signature. If they run out of earned merit, they lose their signature until they earn more.
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I quite like the idea of diminishing sig spamming by restricting signatures to people managing to acquire merit. Do anyone see a problem with that? It would at least reduce spam while allowing anyone who just wants to talk do it.
I like o_e_l_e_o's proposal a bit better, but with more time allowed (e.g. 3 activity period at least) to earn that merit, because if you're on a holiday or have something to do in real life for a while, you should not lose your merits because of inactivity.
Or, if we count only those activity periods, when the user has posted at least 1 post. This would not burn the acquired merits if someone is inactive.