This is probably the best thing you can do if they're using third party "hosted" sending infrastructure - the host will carefully monitor each tenant's spam rate, and terminate relationships with customers that have a high number of spam reports (as it harms their deliverability as a provider).

It's not ideal, but it should start to impact their ability or willingness to send out cold unsolicited sales messages.

I do these for every site. If I make an account somewhere, and I'm sure I unchecked email promotions and still get an email — you get reported as spam.

And for those that have a Github presence, to Github as well.