I don't think that banning someone who has ">x activity and 0 merits" is necessary.
For the most part, these types are not participating in the "Discussion" forums anyway, they hang out in the cesspit of "Altcoin [ANN] land" and "Bounties (Altcoin)"... posting their weekly Twitter and/or Facebook links... and spamming away on Twitter/Facebook/Telegram to earn their "stakes" of whatever various ShitCoin currently needs shilling.
I suspect the reason why all these users are only doing the Twitter/Facebook thing... is that it is much easier to get 1000 Twitter/FB followers than it is to get 10 merit.
If you suddenly make merit compulsory for just continuing to exist on the forum, they're going to flood the other parts of the forum either shitposting hoping to get merit... or shitposting whining that they can't get merit.