how can I disable face recognition?

Are you asking about the automatic scan for faces and the object recognition built into the Photos.app (since you are asking this question in the Photos forum)? You cannot disable this feature in Photos for iOS. It has been possible in Aperture and iPhoto on the Mac, but not in Photos iOS or Photos for Mac. Photos runs the object recognition and the face recognition automatically, as soon as a photo is imported to the Photos Library. The scan for faces is necessary for the automatic classification of photos according to categories. If you are worried about having the faces named, do not assign any names to the detected people and remove all people albums from the main people album. But you cannot avoid the huge amount of processing time to run the object recognition and face detection.