There is a way to sort of kill Faces and keep iPhoto from creating the small faces image files (and to clean them out of the library). It won't stop iPhoto from doing scans but after the first scan it's not much of a problem.
1 - open the library like this in the Finder.
2 - in the Database folder move this file to the Trash: Faces.db.
3 - close the package/folder and launch iPhoto. Close it after a moment.
4 - open the package, find the new version of the file and lock it via the Info window for the file.
5 - while viewing the contents of the package type Command+F to start a search.
6 - set the criteria of the search to: Kind is Image and Name Contains "face".
7 - select all of the files in the search results window and drag to the Trash.
8 -close the package and launch iPhoto.
You will get the faces spinning wheel as it tries to index the library the first time it's opened but it won't be able to write to the database file if you should try to add a face and won't create any new jpeg faces files. Of course make a backup of your library before trying this and no guarantee implied.