Suppress pop up "This operation has been cancelled due to restrictions in effect on this computer"

Suppress pop up "This operation has been cancelled due to restrictions in
effect on this computer"

I've implemented a new Group Policy infrastructure at a school. I'm at the
final stages of tidy up before go live next week but there is one thing
thats got me stuck.
As expected I've locked the students out of most of the fun stuff, but
when they try to do something I have restricted (WIN+R) for example, they
get the pop up message:
"This operation has been cancelled due to restrictions in effect on this
computer. Please contact your system administrator"
Which is expected behaviour as I have restricted that operation.
However I'd like it to just silently not do anything rather than pop this
up (with associated annoying beep that I can imagine kids abusing).
All client machines are on Windows 7, PDC is 2008 R2. Domain functional
level is ahem Windows 2000 (which is on the to-do list before you say
anything).
Any thoughts on how to supress this warning?