I couldn't find a quick answer after a spot of googling for the following problem: how to delete a face from Nintendos Face Raiders on the 3DS.
I found the solution after a spot of prodding the buttons: what you have to do is choose Face Collection, choose the face to delete, then press and hold X then press A.
Now you have access to various customisation options, including delete.
I know - it's probably in the manual. But who reads manuals?