You may hide your main window with iframe defined in start status facet for example.
Thanks but I can't use Iframes. Do you see any other, more flexible solution??
You can look at any "modal dialog" examples on inet. Actually, the using technique is pretty similar. Putting element on the top of the other elements on the page and disable activity with tab. However, if you need to use old IE (6.x and below), using iframe for "disable" the drop down list is still required.
Another possible solution is disabling or hiding input elements and restoring them then.