I had the same problem as you with the show desktop buttons. I googled to fins a solution, but didn't find any.
I figured it out, though, and then i rememberred reading this post. If you still want to remove the Show Desktop Buttons and have been unable, here's the solution:
simply remove this snippet of code.
<Control id="-1" filename="libshowdesktop.so" swapCommands="1" showTooltips="0" lessSpace="1"/>
restart the wm or do an xkill on the panel to force the panel to be reloaded.