Didn't setup offline files, but that didn't stop XP

My users have not specifically gone and setup offline files, but on a
majority of the computers I have they are now syncing up offline files. I
don't want this.

In the users account we map a Z: Drive to \\Fileserver\%USERNAME% and set
user profiles to map My Documents to Z: so everyone stores their data on the
network. We notice that these are one of the folders that are syncing
automatically as well as things in My Network Places.

I do have some laptop users that do sync up so I don't want to affect them.
I know I can use a GPO to disable offline files, but users of offline and
non-offline are scattered throughout my containers so it would be too
difficult to have the GPO do it.

Is this something that is just on by default and is there a way to fix this
without messing up my laptop users?