See this post: http://forum.slitaz.org/topic/disable-openbox-menuthis post
. You have to modify /etc/xdg/openbox/rc.xml and "smartly" remove all the tags matching a "ShowMenu" command. This will completely disable the OpenBox menus (all of them), but you can adapt it to your needs, as the file contents are pretty intuitive.
You can also modify a copy of this file under ~/.config/openbox/rc.xml to make the changes only for a given (set of) user(s), but being familiar with the thin client world, I think you want to remove it completely.
Regards,