Let me start by saying that if a PM is "sneaking in scope creep" he is a very bad project manager.
Having said that...it's not your job to say no to a project manager. It's your job to ensure that he knows and understands the costs and risks of the change he is making. If the PM insists on changing the scope and adjusting nothing else in the project, get another job (because the project and/or company is doomed).
