Once you are a global superstar, you're at the top of the fame ladder...I don't think you get more points to spend at that point. Fame Points, which you spend, are not the same as Fame that you earn by doing things.
Earning Fame adds to the progress of the crescent shaped meter on the right side of the Public Image section of your simology panel. You get Fame Points, to spend, when you fill the gold stars along this meter. Once you are a Global Superstar, you have ALL of the golden stars filled, so it just makes sense you aren't going to get any more fame points. You do still need to keep earning fame though, or you will lose your superstar status.
