Disabling intellimirror on My Documents when redirected to network folder

Hi...

I have a little domain with 6 Windows XP and 1 Windows 2003 Server...

I have redirected the "My Documents" folder of my computer to an UNC folder
on the network (to allow backup,...)... but Windows XP automaticaly
activated IntelliMirror (Offline folder setting) on the My Documents,
forcing a really long syncronisation at every logon/logoff (My "My
Documents" folder contain more than 20Gb !), and multiple errors about
multiple files that cannot be synchronised (.pst files, .mdb,...).

I would like to disable this function, but i cannot : the option is grayed,
Windows XP automaticaly active the Offline Folder setting when a shell
folder like "My Documents" is redirected to the network, and refuse to
disable the setting...

I've looked on the web, and i've found that i can disable IntelliMirror
using a domain group policy... great, but i do not want tot totaly disable
this function, i just want to configure it myself on some folders, and not
on the whole My Documents...

Can someone help me finding a way to disable this automatic feature and
switch to a manual setting ? Is there maybe an option specific for the
intellimirror of shell folders redirected to a network share ?

Thanks in advance...