So you've been syncing a faces a bit too much and you want to clean up so old faces on your watch? Easy, just follow these steps:
WearOS: long press on your screen, swipe to the face you want to delete, and long press again. A red delete icon will appear, tap it, and voila!
Samsung Tizen: triple tap on your screen, scroll to the face you want to delete, and tap the little "X" icon at the top right of the face. Voila!