The book goes into this a little bit more, but...
Unlike an adept (where the magic comes from their own internal mystic viewpoint), a questor's power is ultimately external, granted by their Passion. Falling out of favor (and losing access to Devotion Points) is typically the first step. If they continue to misbehave? That which was given by the Passion can be taken away, up to and including the central Questor devotion itself.
One big aspect of the questor's path is their relationship with the Passion--which honestly is best handled through role-play. If a questor decides they no longer wish to be a questor? They can forsake the path. The difficulty in doing that largely depends on their previous relationship with the Passion, and how much story the GM wants to create out of it. A character who was questing for Vestrial (for example) and no longer wants to? There's loads of story potential there.
If a character is set on no longer questing for a Passion, they can stop. It might not be easy -- for many questors, their identity is often wrapped up in their quest. You might think of it like leaving a faith or converting from one religion to another. Or... a divorce after an intense (and possibly long) marriage feels like another good analogy. There's another person involved (as much as a Passion can be considered a "person), which brings along a new set of challenges to face.
Mechanically, it involves losing access to powers because the Passion takes them away.