Context menus, disabling based upon keyboard state

Hello, I'm fairly new to the use of context menus. While they are extremely handy, I can't seem to find a way to temporarily disable one. The basic idea is that by holding either the shift or alt key the use is able to change the context of the mouse events. The problem is, the context menu appears on a right click no matter what. Is there a way to temporarily disable the context menu, say on a key down event, then re-enable it on a key up event? Thx in advance.