In your group policy editor, go to Users|Windows Settings | Software Restriction | Make New Policy | go to additional rules | new hash rule > and choose the executable that you want to block. Now apply it to the ou with non-admin users.
Additionally, you can prevent messenger itself from running without software restrictions (they can be nasty if you chose to set the default to “restricted” because then you have to authorize paths, hashes, and/or executables to run.
In group policy the setting is:
User Configuration/Administrative Templates/Windows Components/Windows Messenger and the value is <i><b>Do not allow Windows Messenger to be run</b></i>