Using a ducktape.
How autofeed works: Every time, you turn off/on the trimmer, the centrifugal lever pushes the string forward. This results in excessive & unnecessary waste of the string. The video also includes: How to manually feed string in seconds with autofeed disabled.

A user JonJon Taylor recommended:
"Instead of using duct tape or gorilla glue to immobilize the lever, try winding the string in the OPPOSITE DIRECTION from how it was designed to operate. This way the centrifugal force has no effect on the string. I just stumbled onto this method to avoid wasting the line. THIS MAKES IT A MANUAL FEED!I also use the thickest line that will fit thru the hole. I even tried .155 but that was too fat. .105 works good."

Thank you.