The context menu (that you get when you right-click) is built into Flash so you can't exactly disable it.
You can customize the About Xcelsius link though by modifying a file in the Xcelsius Designer, look in the Xcelsius 2008 SP3 User Guide and search for RightClick.txt.
Was looking for the same solution for Dashboard Design which is different to Xcelsius 2008.
The solution is the same as Jeremy S posted above, the location of the file is just in a different spot.
You should be able to find the file in this folder:
Program Files\SAP BusinessObjects\Xcelsius 4.0\assets.
Amend the rightclick.txt file with the options you want and that's it.
Hope that helps.