Scope creep can come from many sources. The project sponsor may be a significant source since he/she probably gains a better understanding of what they want while the project is in progress. Scope creep can also be caused by vendors (they point out something and urge it to be completed now rather than later), team members, and other stakeholders. Project managers need to be flexible to a point, and as a consultant, these often offer financial opportunities in the form of extended projects or a "phase 2" that is developed while the project is in progress.
I never said no. When one of these "ideas" pops up, I document the idea, get estimates, and present it to the project sponsor for consderation and approval before proceeding. Otherwise I might exceed budget and time and no one would know why, which would of course lead them to blame me as PM.