The Managemylife web site is a good resource to find out what may be happening with your Garage Door Opener. I did some research for you at Managemylife.com and found an excellent article that also has multiple expert answers at the bottom of the web page that addresses the most common questions that have been asked by other members. I hope this will help until your expert can answer your specific question. I attached the link below.
Locate the smart/learn button on the motor unit near where all those white wires go. Press and hold this button for 6 seconds. This will erase the memory and no remotes will work. Now you must program the opener to your remaining remotes by pressing the button on the remote and then pressing and immediately releasing the smart/learn button.
I am assuming from your question that someone stole the garage door opener from your garage. To secure the door, you could drill a hole through the edge of the garage door and the rail the garage door rollers fit in. This will require an extra long bit probably. Drill a hole on both sides of the garage door and buy 2 extra long padlocks to lock the door to the rail.