Disable "messenger" service via group policy?

I work at a school that needs to disable the messenger
service for all students(they have been netsending
messages to the domain despite having the command.com
disabled). How can this be done? Currenlty I
have "svchost.exe" entered into the "Don't run specified
Windows Applications" part of the group policy. Any
ideas?

If I posted this in the wrong forumn, I apologize.

Thanks