Before entering the chat, there is an option to disable it, by selecting "I won't use webcam" from the dropdown.
If you do join with the webcam enabled, it will display your camera for you. The other users in the room will not be able to see you unless you grab the mic. (There is no way to know if the site/administration is recording/viewing the camera when you are not using the mic)
If you want finer controll over your webcam, you can install a camera program that will allow you to manage it better, such as Youcam, Quickcam, or Splitcam(free)
You just need to go to the flash settings and select Splitcam instead of 'Integrated camera'
When in doubt, use electrical tape. It will prevent anyone from seeing you unless you remove it. For security reasons, it is a good idea to cover the camera when not in use anyway. :]