Disable Outlook offline use?

Can anyone tell me how I can disable Outlook offline use; the 'Disable
offline use' button in the Offline folder file settings configuration screen
is permanently disabled.

The problem is we have one user who sometimes connects remotely through a
hardware VPN but with Outlook insisting on creating a .OST file of 170Mb
every time this is seriously slow. We are using roaming profiles which
enables this user to get the same desktop whether in the office or at home.

Nick