How to prevent password reset using OS X installation disc?
Mac OS X (and now OS X) has always allowed the installation tools to reset administrator passwords.

Rather than locking the OS which can always be modified, you can either encrypt the OS image using File Vault or use a Firmware password to prevent other OS from booting. Many people use both tactics together for enhanced security.

Setting a Firmware password is straightforward, documented in the system help and can be done via the installation disk or Recovery Partition.