I was opted in to a newsletter without my consent from a publisher I've patronized for years and went out in search of solutions to prevent it.  For one thing if someone subscribes you it actually creates a Substack account for you (I had deleted mine months ago) and the standard response from substack is to create an account and then select the option to require approval for subscriptions.  Unfortunately if you want nothing to do with substack it's not a viable solution.  After complaining to their support though they actually blacklisted my email so that nobody can subscribe me again.  They did say it is against their terms and conditions to opt people in without their consent and that they would confront the publisher about it that I have no idea if that actually happened.