Hi Jesse,
hmm... good question and a nice feature request :-)
Technically, if an object gets shared, a new NetGroup gets created for just this object and all peers who want to receive this file then join this NetGroup and request the object. The problem is that every peer that has received that object (or a chunk of it) automatically becomes a seeder (publisher) for that object as well and will act as a "hub" as well. So, to effectively remove an object from the NetGroup all peers in that NetGroup need to call NetGroup.removeHaveObjects() passing in the indices of all received chunks so far - also, you'll have to tell the ObjectManager to remove all references to the ObjectMetedataVO from the internal data structures dataDictionary, senderDictionary and receiverDictionary (these are actually wrapped by the sharedObjects / receivedObjects ArrayCollections). You could trigger the removal by sending a custom message to all peers.
No promises though that this will work.
Dirk.