I don't believe this is possible, and here is why:
The Forms Server tries its best to do everything in the browser without sending data back to the server (aka postback). However, there are times when it has to be done. Any time it has been determined that a postback has to happen, Forms Server has to block the user from making any edits until the operation has completed. This is done so that we prevent data loss if you have typed something while Forms Server is waiting for the postback to finish. The dialog you have mentioned appears automatically in this case.
Having said that, it is important to realize that the dialog only happens when you are sending data back to server and it is a good practice to avoid this in general. When designing the form, use the Design Checker with the "Verify on server" option checked to see what controls are posting back and why. It is possible to reduce the number of postbacks or to eliminate them completely with this method. Also, you can tell individual controls never to send data to server automatically by chaging the settings in the "Browser Forms" properties. Note that by doing this your form might behave incorrectly!
I hope this sheds some light on the issue,
David.
David Airapetyan / Software Services ArchitectQdabra® Software / Streamline data gathering to turn process into knowledge