And in my mind, one email alert or pop-up to all current users letting them know the service is going away (in case they had not been told which is as I've said before often the case in large orgs) is very far from the definition of spam.
If this happened in real life, I'd say "Joe," (because I'm on a first-name basis with the CEO and chat with him daily), "we had to block communications from Tom's company because they're pulling some bonkers stuff. You might let him know that he's pissing off customers right and left and needs to knock it off."
And yes, this is 100% the definition of spam. It's literally unsolicited commercial communication. Sending these mass instructions to flood the local IT admins is an enormous breech of trust.