You can disable the camera app with:
dsmetool -k /usr/bin/camera-ui

And turn it back on with:
dsmetool -t /usr/bin/camera-ui

This doesn't actually disable it. It disables starting the camera app on lens open event. You can call the camera app manually through DBUS if you want.

If you are working with Qt you can override your form close event, to turn camera app back on, when you are done with your app. Don't know about GTK, but it should be pretty similar.