Set up a WhoAmI web service that gets the logon ID of the currently logged
in user. When the form is being submitted, save that logon ID somewhere in
the main data source. This will get you started:

Set up a view where all the data is read only. Make sure this view is NOT
the default view.

Create a rule that fires on load of the form: if the logon id of the current
user != the logon id stored in the main data source, switch to view that
presents read-only version of the data.

Good luck,
Alex