Th motherboard battery only controls the password function in the BIOS - it doesn't sound like this here. If you take the battery out yu will lose all your BIOS settings - so don't do it unless you know why, and how to resetup the BIOS.
It's very difficult to bar anyone from accessing the computer jut by using a password at the start-up screen. It's simple to at least partially get round just by closing the password window.
To truly stop a casual user getting into the system, you could try a password in the BIOS settings. This will stop the system from even booting up, and can only be gotten around by taking the case off, etc.