How do you prevent scope creep on a MERN MVP?

Every feature request during the MERN MVP build either fits the locked scope or moves to the written post-MVP roadmap that we maintain in real time. Founders can see the roadmap growing as features are deferred. Scope changes to the locked MVP scope require a formal change request that re-prices and re-timelines the build. Most founders stop asking after seeing the roadmap grow in real time, because the deferred features are not lost. The discipline is what gets the MVP out the door on time.