System 7.5 includes a Control Panel called Button Disabler that can be installed with a custom system software install. This control panel has a single check box that, when checked, deactivates the sound and brightness controls on the front of Macintosh computers with built-in controls.
Use the Custom Install feature of your Macintosh CD to install the Button Disabler control panel, restart your computer, and click to place an 'X' in the box to the left of "Disable Buttons".