If you need to get back to getting the Exit button to give you the exit menu, go into MediaCar setup, and change it back to Ask There is just the 1 button which gives you shutdown/exit, if you hard code it, then it will always do that. I just use the menu for ease of use, it's only a couple more seconds to hit Power Off once i've pulled up the menu
As for removing un-wanted buttons, go into the C:\Program Files\MediaCar\SKINS folder, and in the folder for the skin you are using, find the .skin files, you will probably want "Menu.skin", and open it in Notepad.
When it's open, put a / in front of any lines for buttons you don't want. They should then not be shown until you go back and remove the / from the line I am doing that with the Radio and DVD buttons on mine