How to prevent Alt+Shift from switching keyboard layout before keyup?

Motivation: In an appplication I'm using, Alt+Shift+R is taken up by -- well, never mind what, something app-specific. However, on my Linux system (Kubuntu 12.04 specifically), when I press Alt+Shift without depressing any of these two keys, the keyboard layout changes.

Question: How can I make the keyboard layout change in X only take place if I depress Alt and Shift, but not if I press another key before depressing them? So that Alt+Shift+R only affects the application in my example?

Possible workaround

Go in Settings -> Text Entry. Change the keyboard shortcut from Super+Space to something else (ex. Ctrl+Space).

Good luck