Stop Super+M from opening Messaging Menu
How can I make Super+M and Super+S work for compiz effects and not for indicator applet & indicator applet session in Lucid?
Unfortunately the shortcuts for both indicator-applet and indicator-applet-session are not configurable, which causes problems. See bugs #558581 and #554713.
If you are brave and know what you are doing, you could use one of the patches listed in bug report #558581.
You could also use indicator-applet-complete
which only uses the Super+S shortcut. See this answer for more details.