hey!
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:
edit ~/.xfce4/xfce4rc
simply remove this snippet of code.
Code:
<Group>
<Control id="-1" filename="libshowdesktop.so" swapCommands="1" showTooltips="0" lessSpace="1"/>
</Group>
restart the wm or do an xkill on the panel to force the panel to be reloaded.
Cheers!