How do I actually disable F11-fullscreen on Lubuntu 13.04?

I think just deleting the lines for F11 may not serve the purpose.

You need to replace the contents after action name= to nothing.

So, change action name="ToggleFullscreen" to action name="", save the file, and then run openbox --reconfigure in a terminal. There should be no error message if your lubuntu-rc.xml is in good condition! There's no need to reboot. Now, F11 should have no effect as far as Openbox is concerned.

I don't know about your other programs which also want to use F11 for something else. For example, both Firefox and Google Chrome will still use F11 to toggle full screen. On the other hand, F11 in LibreOffice will now function to open the "Style and Formatting" panel.