It is actually possible to delete a user from the Registry, once the user has been removed from all Paratext 8 projects. However, to do that, you need to have an administrative role in the organisation that the user is listed under.
The user can also remove their own account (again, once they have been removed from all projects in Paratext) by going to the Settings page (which can be found by clicking their name in the top-right corner) and clicking the big red button at the bottom of the page.
The feature whereby users can change their registration name will hopefully be pushed to the live server next weekend.