The apps that can successfully keep other apps from accessing the camera on your device use a Device Administrator to do so.
Basically, this gives the user a prompt to allow this administrator, and then that app can cause the camera to be unavailable.
The camera will appear to be nonexistant, or already in use by any other USER app that tries to access the camera.
I emphasize 'user' apps, because it is not known if system apps or other built in apps can bypass this restriction.
After I typed the above, I went to three different app stores to get the link to the app that I use, so that you could check it out.
It seems to have disappeared.
It is called Pandroid Camera, and works just as I explained above.
As I cannot find the link, I could upload the apk to cloud storage and then post a link to that.
I use this app on all of my devices, and highly recommended it.