It's important to note that with Family Settings enabled, the tiny
bit of handshaking that may
occur does not affect your machine or profile.
Also, you could just yank your ethernet cable out whenever you sign in. Problem solved.
However, to answer your original question, you need a program like Modio or Le Fluffie to edit the Profile ID that each gamesave is tied to.
I haven't personally done this, but it doesn't seem difficult at all to do. I'm sure there are tutorials on doing this as well.
I'd suggest you lurk Se7en Sins a bit. They usually have crap like this lying around.