After a reboot, owner always has to be the first to log in. If switching profiles after that is not allowed, you can't use any other profile other than the owner profile, which defeats the whole purpose.
If you're worried about multiple users running at once for performance or battery reasons, just end the profile's session before leaving the profile.
If you don't want people to switch profiles, use unique and hard to guess PINs/passwords.
Or maybe you have another reason for wanting this functionality?