American Express wrote it: We may also cancel this Card account and other Card accounts you may have with us, but shut you down might also refer to blocking future bonuses.

I don't think we have any reports of accounts closed for 12 month violations, but we do have reports of posters who are not eligible for bonuses. It's hard to know for certain if 12 month violations are the proximate cause because American Express cites only a list of activities, not specifics. In any event there is no compelling benefit to closing an account early, unless you are up against the 5 card limit and urgently need another product.