Short answer
HubSpot's KB: removing someone from an account stops their access, but the user tied to the email address still exists. You can permanently delete your own user when it isn't associated with any other HubSpot account. Deleting it also deletes Academy certifications on that email, so transfer them first.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
A user's default meeting link can't be deleted or transferred. Ask them to turn off scheduling pages before deactivating.