There's not specifically an easy way without a bit of scripting on your part. I attempted to write you a quick VBScript that would use the WMI_SystemDriver class to enumerate all devices that contained the word "camera" in the description and then stop them.
The problem is, my Lenovo webcam doesn't allow that call; the driver exposes itself to the OS as 'unstoppable' in terms of user service control. I have a suspicion that a lot of webcams may work the same way. To get around this, you need to make a DLL call to tell the OS to disable the hardware device, which cannot be done in VBScript.
I could certainly write you such an applet, but then you'd have to simply trust me rather than being able to see what was inside - unless you have access to a C# compiler and just want the source code to compile yourself.
Another alternative that requires a lot of download time but then would allow you to use a simple DOS command to do the toggle is to grab the Windows 7 DDK from Microsoft. Part of the DDK is a utility called Devcon, which is a command-line version of WIndows device manager. A little bit of DOS wizardry with the FOR /F "usebackq..." command to parse a devcon status statement followed by another devcon enable or disable statement would allow you to toggle the hardware state and would get you where you want to be. But it's a huge download for a single stupid file, unless of course you already had use for the WinDDK.
