How to Delete Custom User Menus?
Currently, the only way I’m aware of is to restart ZBrush without saving the UI. Or if you have previously saved a configuration that has custom menus and now wish to remove them you will need to delete your CustomUserInterface35.cfg and create a new custom UI from scratch.
I personally recommend copying your custom UI file before you start fiddling with creating custom menus. Save your UI when you’re happy with a menu (and copy the new file so you can always revert if you want) before moving on to the next one. This will avoid the frustration of having put a lot of time into building multiple menus only to change your mind or run into a problem with the final one.