Most BIOSs can be configured with a "configuration password". This should set and tested. Some motherboards will require a jumper to be set to allow the password to take effect. Some BIOSs have well-known "master passwords", use a search engine to ensure that your BIOS is not one of these. The password should not be the same as the boot loader or root passwords.
The BIOS configuration will have a "boot order" setting. It should be set to boot from the hard disk before any other media. This prevents someone inserting a rescue diskette, booting the machine, and gaining access to the filesystems as root.