I was called out on this a month or so ago by auditing for 2 multiple night stays. I was doing the same thing as you and guess how they let me know !! I went to log in and my account was frozen with a note to call Marriott Rewards Customer Service. Customer Service, transfered me to auditing. The guy in auditing asked if I had received my "letter" yet, which I had not ... He went on to say what I did/tried to do was a major no-no and that they were specifically targeting this "kind of abuse". I apologized, said it wouldn't happen again and he turned me back on. Soooooo, be careful. By the way, when I got home that "letter" was waiting for me, explaining in detail what the rules state. Didn't get the stay (obvious) and didn't get the points either.