How to disable panel context menu from locked down panel (help and about) - for kiosk like user account


I’m trying to create a near kiosk mode user account. We’ve been using gnome-flashback which worked well through 16.04 but on 18.04 there are all sorts of problems with the Nautilus desktop especially not being able to turn off “have the file manager handle the desktop.” The “solution” provided by the flashback family, in their own words, has similar problems as Nautilus (they proposed using Nemo to handle the desktop but it has the same failings). They are working on shell extensions but I haven’t been able to doing anything with what’s been put out there so far.

Mate is great and I’m proposing to use it in production but I have a similar problem. I can lock down everything for our limited user account except right clicking on the panel. The help and about entries open up doors - particularly the about which can give you links to set up all sort of things (which may not work for permissions reasons but it’s ugly).

Is there a way to disable the context menus or right click on the panel (or edit the items presented)? Such a minor detail has become a major blocker.

I appreciate any advice. Thanks very much.

Edit: It’s the mime handler that opens up Thunderbird when clicking on one of the developers names that starts opening up holes. I removed Thunderbird so the click does nothing. Turning off URL and MIME handlers in dconf-editor didn’t work. This test system originally had gnome-flashback, which I uninstalled but I wonder what was left behind. Had a screen lock issue too - which I suspect came from the gnome screen saver. I think we’re going to go with Mate but that right click on the panel is awkward and may cause confusion for end users who have never used a linux box before. Ideally I’d like to suppress the “help and about.”