I've finally figured out how to do this but it involves getting into one of the SQLite databases iPhoto maintains. Definitely back up your iPhoto library before doing any of this.
First, download and install MesaSQLite.
http://www.mesamysql.com/?realmesa_home. This is what you'll need for working with the database file. It's free. There's other tools and methods for working with SQLite DBs but this is how I do it.
In Finder navigate to Pictures and right-click on iPhoto Library (or wherever your iPhoto is located at). Select 'Show Package Contents'. Find and open face.db with MesaSQLite. Select the "Custom Query" tab at the top.
In the text box enter the following:
select * from face_name where name is not null;
Press the 'Run Query' button. This should list all the faces you have ever named. Even the ones you have since unnamed and unfaced. Scroll through the list and note the 'face_key' of the names you want to remove. For each name you want to remove enter the following in the text box and press Run Query.
delete from face_name where face_key = <face_key>;
where <face_key> is the numeric face_key you noted earlier.
Be really careful! Accidentally deleting names which are still active in your library could have unpredictable results. Please please please backup your library before thinking about doing any of this. I'm a software developer who has worked with SQL databases for over 15 years so I'm fairly comfortable with this kind of thing. If any of this makes you nervous you may be better off just dealing with the extra names and hope the next version allows for better management of orphaned face names. But if you really absolutely have to have those names out, this should work.
