OK let me be more specific.
Instead of making your subFrame extend a Panel let it extend a JDialog. In the Constructor of the subFrame, you just call the super's(JDialogs) constructor with 'modal' argument true.
Here parent is a JFrame variable which you will pass as 'this' from your main window.
This will make your new window modal ie, the control can never be passed back to your parent frame, unless the JDialog window is closed. So it will restrict the user from clicking the 'open' button again when the new window is open.
Hope this clears the matter and hope this solves your problem.