Hi,
There is a workaround solution for your requirement. You put one text box on the form. add one more node in the datasource but do not include it on the screen. (name it as my:MakeREADONLY) this will work as a flag for making the first control read only. Now add conditional formatting on first control that if the flag is true then make it read only/disable.
Programatically you set the value of the flag field to TRUE as per your requirement.
XPathNavigator
ReadOnlyNav = MainDataSource.CreateNavigator().SelectSingleNode(
"/my:myFields/my:MakeREADONLY", NamespaceManager);
ReadOnlyNav.SetValue("1");
Hope it will solve your problem.
Bhavesh