I received the same spam email to the forum-specific mail I used to sign up, so I know for an absolute fact that it was sourced from these servers.
I'm from the EU, and I did not actively opt in to ads from BikeForums.net, or anyone else for that matter.
That's a GDPR violation right there, and means the owners of BikeForums are liable under those laws. Now, how to proceed with this?