How do I disable Compiz in the Ubuntu Classic session?

The easiest way is to use the session that's there to do exactly what you want. ;-)

• Log out.

• At the GDM login screen choose the Ubuntu Classic (No effects) session:

  

• Log in.

  → You will be in a classic GNOME session without Compiz. Subsequent logins will use this session as default.

If you don't have some other reason to believe that compiz is running (double check with ps aux | grep compiz), then it sounds like your real question is "what's the relationship between gnome-wm and metacity?" or "how do I turn off shading and effects in Natty?" (Both very good questions, if you ask me.)

You would add metacity --replace at startup applications.

1. Open it

   gnome-session-properties
   

2. Add

   metacity --replace