If you don’t want the built-in webcam to ever be activated, use the linked file below.
My previous method of disabling it completely doesn’t work any longer in El Capitan without turning off SIP, but this does.
Note: no security method is foolproof but this requires quite a few steps to get around.
It’ll add a Device Profile that prevents the camera from turning on no matter what application — like FaceTime — attempts to activate it.
Of course you can put black electrical tape over it but I’d prefer to not get the resulting goop on my iMac.