Seems to be quite a common problem with Fiesta's maggie.
Unfortunately, this is not an option on your Fiesta. Other than replacing the door mirror, you have two choices: either disconnect it or try to reset it. To reset the mirror, take the following steps:
Remove the small trim panel on the inside of the door, behind the door mirror; this should gently pry out of its fixings. You will then see the loom multi-plug connecting the mirror.
With the door closed and the window down, fold out the mirror as far forward as it will go, disconnect the plug and lock the vehicle using the remote. Then reconnect the plug and the mirror should motor back to the closed position.
Disconnect the plug and unlock the car, fold the mirror back to the forward position, and now lock the car and reconnect the plug. This time, the mirror should wind back to the normal driving position.
Disconnect the plug again and unlock the vehicle. Fold the mirror to the closed position, then reconnect the plug and lock the vehicle. When you unlock the vehicle, the mirror should then motor to the correct position.
Now, when locking the vehicle, the mirror should fold correctly and stop without the flapping.
If this does not work, the other option is to leave the plug disconnected; this will also prevent the remote adjustment from operating.