Restart your Mac, hold down Command-R when you hear the bong.
When you get the Utilities screen, go to the menu and choose Utilities/Terminal.
In the Terminal window type: resetpassword and hit Return.
Use the Reset Password tool to select your disk, userid, and new password.
Notes:
This will not work if you have FileVault turned on.
You will lose access to your keychain, OS X will create a new one.
If you have physical access to a computer, whether it be Mac or Windows, then you are in control of that computer. If you want to make it less easy you use encryption.
You can do the firmware password but if you use Firevalult your hard drive is totally encrypted so it cannot be accessed without the password--or the recovery key that was generated when Firevault was turned on.