The offline button cannot be explicitly disabled without customizing Workspace
I've had this requirement a few times. Kind of a quick and dirty solution I've used is to put some script in the Pre-Save event of your form to hide the top-level subform. Then you'd have to put one in the Post-Save event to show it again. This obviously isn't an ideal situation, especially because the XML bound to the form is still available if the user knows how to retrieve it. It does however save you the hassle of customizing Workspace.
Hope this helps!
Ryan D. Lunka
Cardinal Solutions Group