Everything lives in Account Settings → Family Access: pending invites can be resent, and every member has a Remove access button.
Their access to your vault ends immediately — enforced on the server, not just hidden in the app. Their own NestVault login still exists, but it no longer opens your vault. Removal is permanent in the sense that re-inviting them is the only way to restore access.
It can’t erase what they know. A removed family member still knows your vault password — and if they ever saw it, possibly your recovery code. Removal closes the door; it doesn’t change the locks.
So when a removal isn’t entirely amicable — a separation, a falling-out, or any situation where you wouldn’t hand that person the password again today:
The full sequence, with what each step does and doesn’t change, is in I think someone else knows my vault password.