This is a tiny portion of black metal in a sub genre . But it isn’t surprising that an extreme genre with a strong counter culture doesn’t have some extreme spin-offs.
Also you can tell which bands are in NSBM because they are all terrible at actually making music.
I mean Amazon did this for their mp3s. It was literally just an id3 tag with a unique identifier. Not hard to remove but “good enough” to keep regular people from overly distributing it. You’ll never win against the real Pirate community no matter what you do, so just give people real incentive to buy and actually own.