Wow, I have always heard the threats (from my corporate travel agent) about airlines enforcing throw-away ticketing, but have never heard of an example where they actually do.

Congratulations, OP! You seem to have gotten yourself in the worst possible situation -- repeated infractions, a major price difference, and a highly visible scenario. I don't have first-hand experience, but it's a fair bet that if they sent you a letter, then they will continue to take this seriously.

Hopefully they will just deny you the miles instead of worse outcomes -- e.g., cancelling your frequent flier account or issuing debit memos.