locking the panel to users

[Alvise]

Hi everyone,
	first of all I'd like to thank the xfce staff for their amazing work: I 
have been using this since quite some time (maybe the first xfce 
release, if my memory is right) and it just got better and better.
I am planning to use xfce4 on many computers in a library that need to 
be "locked",i.e. they just have to let mozilla run.
I looked around to understend how to do this, and got into 2 steps: 
modify and chown the user's menu.xml, and lock his panel via 
XFCE_DISABLE_USER_CONFIG.
Here's my problem: setting the enviroment variable I achieve that user 
cannot modify elements in the panel, but I can't get rid of the "Add 
item" in the panel's menu.
In truth, if the user adds a launcher it will be gone in the next 
session, but I'd really prefer if he can't add anything at all!
I have seen in the Mailing List archives that someone says that setting 
XFCE_DISABLE_USER_CONFIG will hide that menu, but it doesn't seem to 
work for me...Any clue?

Thank you for your attention and help.

Alvise