Does anyone know how to disable items in a control's system menu (the popup menu that shows when you right click on it, eg. a TextBox's menu has items like Paste and Undo)?

I tried using GetSystemMenu and then GetSubMenu from there, but there where no handles to sub-menus in there. So I don't know where to look for the popup menu!

What I'm trying to do is disable (or delete) the Paste command from there. Any help is greatly appreciated.