Two ways to Disable iSight

Apple – Support – Discussions – Disable iSight ….

1.  Use Parental Controls

2.  You do not tell us which Mac and Mac OS you are using.

If you have a built-in iSight and have removed the “QuickTimeUSBVDCDIgitizer.component” file from your computer’s HardDriveName > System > Library > QuickTime folder, restart your Mac and try again. Your built-in iSight should no longer work with any app.

If you are using external (Firewire) iSight, you need only unplug the Firewire cable to disable your iSight.  (Thanks EZ Jim)