Administrators can still manage their own and anyone elses profile by clicking the gear icon next to the user name on the Member List.
This use case is really an edge case so I tried to provide you a quick easy solution, but its not a priority at the moment to put more effort into a more complex soluition for this use case. You can easily now create a demo site with predefined demo users and no-one except admin can change profiles.
What you can't currently do is create demo accounts within a real site without also limiting what other users can do by disabling the profile.
If this use case is important to you, you can consider hiring me to implement a more advanced solution. This use case is not important to me so its not a priority but could become a priority if a paying customer wants it.
Best,
Joe