How can I prevent someone from resetting my password with a Live CD?

Solution 1:

A quick and easy way to do this is to disable booting from CDs and USB sticks in your BIOS and set a BIOS password.

According to this wiki page:

Placing passwords or locking menu items (in the Grub configuration files) does not prevent a user from booting manually using commands entered at the grub command-line.

However there's nothing stopping someone from just stealing your hard drive and mounting it on another machine, or resetting your BIOS by removing the battery, or one of the other methods that an attacker can use when they have physical access to your machine.

A better way would be to encrypt your drive, you can either do this by encrypting your home directory, or encrypting the entire disk:

• How do I encrypt my home partition?
• https://help.ubuntu.com/community/FullDiskEncryptionHowto

Solution 2:

Stand next to your computer while holding a tee-ball bat. Severely beat anyone who gets close.

Or lock it up.

If your computer is physically accessible, it is unsafe.