Hi Paul,
When you right click on the Spectrum Analyzer block and click on the "Properties" option, the following window will pop up:
After you click on the highlighted link, it leads you to the Spectrum Analyzer1 window which looks like the following:
On the top-left corner you can select Spectrum Settings(highlighted using Yellow) and then you can set the sample rate using the option as shown on the right (highlighted using Red).
Hope this helps!