[jrootabega]

Blocking the sender won't even work. These companies, though they maintain plausible deniability, deliberately ignore and evade attempts to escape their marketing space. They will occasionally shift the domain and sender to counter any blacklists. Or they will find a way to link your previous subscription to one list to a business relationship that allows them to add you to ANOTHER list. The most effective solution would be a fuzzy match on the org's name, but you would probably lose legitimate messages that way, and they also like to change their names every now and then. I'm not talking about sketchy criminal companies, but large US-based companies and charities like OP is.

The only thing you can do is consider it a kind of digital chicken pox. It will always be a part of you, but hopefully it will remain inert. Never do anything to acknowledge that you have seen the messages or that you even remember that the sender exists.