How can I prevent FaceTime from launching when I plug in an external video camera?
You can control which applications automatically open after attaching a camera to your Mac using the native Mac application, Image Capture. Try plugging in you camera and then launching Image Capture. If the camera is recognized by the app, there will be a drop down menu visible in the bottom left-hand corner of the app. Click the drop down menu to choose which application to automatically open when the camera is attached (in your case, choose "No Application"). 

If for some reason Image Capture does not recognize your camera, the drop down menu will not be visible. In that case, I'm not sure how to fix your problem.