Welcome to iMore! If you're looking for a way to disable your iSight camera without installing additional software you'll have to enable Parental Controls for your existing user profile on your Mac or create a new one with the controls enabled. Here's how:
1. Open System Preferences
2. Select Parental Controls
3. Decide whether to enable Parental Controls for your existing user profile or set up a new one.
4. Under the Apps tab, deselect "allow use of camera"