No.

From what I understand of the FastTrack network, it's still server-centric meaning that all clients must connect to a central server of some sort. Changes to that one server could then potentially break the clients accessing it (that's apparently also how Morpheus was locked out of the FastTrack network).

But because the Gnutella network is pretty much based on open standards and there are no central servers that you must connect to, and the fact that there were numerous clients making up the Gnutella network even before Morpheus got here, it would certainly be a feat for Morpheus to somehow shut any other clients out. About the closest they would be able to come to doing so would be to alter the Gnutella protocol to essentially form their own network. However, they wouldn't be able to talk to the Gnutella network anymore if they did so and the Gnutella network would still exist without them.