I keep getting emails from an organization I was associated with years ago. They do not have an opt-out link. How do I get them to stop?

These don’t appear to be automated email-blasts, but rather updates on the organization and upcoming events from different individuals at the organization who must have my email address on some kind of list.

Every time someone emails me a mass email, I email them back asking them to remove me from their list as well as any other employees lists. A few weeks later, I’ll get an email from the same organization – but from a different person.

How do I get this to stop as tactfully as possible? Would it be rude to call them? Should I be more assertive in my email responses?