Are you using FileMaker 11?
The following solution works with FileMaker 11, not older versions:
Define a calculation field, cStatusFilter as:
If ( Creation Date + 14 < get ( CurrentDate ) ; "Closed" ; Status )
Make this an Unstored calculation or it won't update correctly as time passes.
Set a filter on your portal to limit the visible records to those that are open:
PortalTable::cStatusFilter = "Open"
If you aren't using FileMaker 11, let me know and I'll describe a different approach.