Dezz,
Presuming you are the Administrator…
1. Open System Preferences/Startup Disk
2. Click on the lock icon in the lower-left “Click to prevent further changes.”
Now if the other person has an OS X install CD, they can restart your system with Disk #1, change the password etc. and then regain access to all volumes. While there are prevents for this, for all practical purposes, steps 1& 2 above oughta do it.
Harv