The trick with changing the identity of the NPCs doesn't work for me. What happens is the new identity continues to fight alongside you but refuses to acknowledge that you've ever hired him/her in the first place. When you talk to him/her, you get their "hire me/here's what I do" conversation, which doesn't contain an option to dismiss the character. Once you've done that, there's no way to get rid of the character at all, short of restructuring the file.