So you advise to block entire culture instead of having it on provincial level or because you want it in few tetitoris? This sounds a bit off..
That is because I am a role player. I think there should be an idea behind not allowing pops promoting to freeman, like cultural discrimination.
On the other hand, if you want to min max the game, I would like the devs to implement a building or a policy instead of a button. For the province, it could be a Governor policy: slave land, no matter what culture you are, you will not be able to promote from slave.
EDIT: this policy shall have negative provincial immigration and a meaningful negative happiness to all population on the province, except for nobles, they love their inequality raw.
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