It's only spam to you because you clearly have no interest in having a Flying Blue account. (I don't even understand why you didn't just let your Air France account just die. Your web browser can usually fill in fields automatically for you, so it's not as if the Air France account was much use....particularly if you never fly on Air France or Skyteam!).

We hear from time to time of people whose mileage balances have expired, despite receiving warnings in such "spam" emails. If receiving monthly emails from a frequent flyer account is of no interest to you, then the frequent flyer account itself is of no interest to you - and I would advise you to write to them requesting your account be closed.