I have a membership with 24 Hour Fitness. Obtained on the cheap from Costco, I had first used their three-day pass, which requires one give an e-mail address to obtain it (and of course, means you get their spam thereafter).
I am unsure that their stores' systems and marketing system are the same or not. Though a divorce would be my bet, I do not desire to risk not receive member mailings by unsubscribing to the "become a member" advertisements.
Not that they have ever e-mailed a useful message for members, or any that I can remember, actually: ultimately, if I had to bet on it, I would take the option "block those too". Nevertheless, perhaps they will send a reminder nearing termination of my current membership?
So I within Gmail I went to "Settings" (by clicking the icon of a gear on the upper-right), then "Filters", chose "Create a new filter" (at the bottom of the rest, listed on that page), entered the relevant e-mail address, and in the "contains" field (for text) put
INITIATION FEE and MONTHLY DUESwhich always appear in their "become a member" messages. Hopefully that should end any future contact with their marketing department or outsource-company.