Is there a way to cap the maximum volume in Windows 7?
Yes, it appears that there is a program that can do this for you. It is called, quite appropriately, Volume Lock. It is trialware with a 14 day trial period. It says that it has a password function, so try it out!
In Control Panel > Sound > Speakers > Properties > Levels > Balance the L and R channels can be reduced, which creates a volume cap. However, this solution relies upon this "trick" remaining undiscovered...