It is fairly easy for the Morpheus developers to enable Morpheus to download from LimeWire. There are ways to block them for good (e.g. make it impossible for them to download from LimeWire or to connect to LimeWire without violating the DMCA) but I don't think LimeWire will go down that road.

Gnutella is an open network and as long as that is the case you cannot ban harmful vendors entirely.