That's the way to do it. But on the original post you made it sound like you can turn down a junior assignment and expect the company to say: "oh sure, no problem, it's in your contract, we will just cancel the flight, have a nice day now." What you are describing is ignoring an attempt to junior assign you. Once they get a hold of you (if you let them do so) and you don't intend to accept the junior assignment it gets ugly very quickly.