Mmhmm will be "on" as long as it is the selected camera in the program you are using it with. If you would prefer to not use mmhmm for any given call, simply navigate to your platform's video settings and select your default webcam (i.e. unselect the mmhmm camera).
This will have effectively turned mmhmm off until you select it again as your desired input camera. If you don't do this and mmhmm is not open, you will likely see a staticky black and white mmhmm logo.
If you deleted mmhmm and the remnants of the app are still living on your computer, make sure you download and run the installer to remove the camera plugin.