From the Chamberlain customer support site:
How do I erase the remote control from the garage door opener's memory?
The memory in the garage door opener must be erased to prevent an unwanted remote control from operating the garage door opener.
To erase the memory perform the following steps:
NOTE: Erasing the memory will remove ALL remote controls and keyless entry codes from the garage door opener.
Locate the "Smart/Learn" Button on the back or side of the motor unit; generally this is on the same side as the antenna wire. You may need to remove a light lens cover or open a control panel to gain access. The "Smart/Learn" Button may be Green, Orange, Red, or Purple, and will have a small LED next to it.
Press and hold the "Learn" button on the motor unit until the learn indicator light goes out (approximately 6 seconds).
All previous codes are now erased.
Basic Programming Steps
Locate the Learn button.
Garage door opener: Located on the back or side of the motor unit (you may have to remove the light lens). The Learn button may be green, orange, red, purple, or yellow with a small LED next to it.
Gate operator or commercial door opener: Located on the control or logic board, with a small LED next to it.
Press and release your Learn button.
Within 30 seconds, press and hold the desired button on the remote control.
Press the remote control again when the opener lights blink. The operator or opener should activate. If not, repeat the steps.