Stop dropable for a Particular div

Stop dropable for a Particular div

Hi,
I have a set of sortable portlets as shown in the diagram

Here in this example the portlets move from places to places, but I need a functionality where the Portlet named Feeds should be immoveable and also when other portlets (other than Feeds) are kept over Feeds portlet then that portlet should be moved back to the original position. 
In other words to say Portlet shopping can be moved to positions of Links or Images or news...
, portlets shopping, news, Images cannot be moved to Feeds Location but News, Images, Shopping can move places with each other.