Here's a great external blog I found describing the registry keys to disable the Start button and Done button on Windows Mobile: http://www.hjgode.de/wp/2010/10/11/windows-mobile-hide-startbutton-in-winmo-6-5-x/ . Although RhoElements offers OS lockout capabilities this allows you to have an additional level of control.
I've also attached a PDF version of the blog to this post in case the link becomes inaccessible.
Another great tool for locking the OS functions from users is Motorola's AppCenter available on Motorola Solution devices. Best of all it is free.