Video setting spontaneously changes from HD camera to OBS virtual camera during a meeting

[Steve_PDX]

Even when we don't have OBS open, its virtual camera shows up as a video device option on Zoom.  We have a webcam attached to the PC via USB and select HD camera in Zoom.  For no apparent reason, the setting will sporadically change to OBS virtual camera during a meeting. The Zoom task bar then shows the camera muted icon.  We can return to the webcam by unmuting it on the task bar or changing the option on the settings menu. A little while later it will switch back to the OBS option.  

This is a new problem.  The camera works fine.  We control the PTZ camera via its LAN interface which shows no problem with that video output.

Short of uninstalling OBS, what can be done to stop this unwanted, sporadic change?