Messenger security

I work for a corporation and there are a couple of our users that use
Windows Live Messenger 8.1. We have all the conversations writing to
a file on our server, and going through a virus scan, as they write.
We have sent out documentation stating that we are monitoring this, so
we have covered our bases with that issue. Problem is...We have a
user that has downloaded a virus from messenger. When we looked at
the settings on messenger, we noticed the conversations are no longer
being written to our server, much less going through the virus scan.
Is there any way to make it to where they can not change the settings
back? We are just concerned that the users will try to keep changing
them back.