I tried the jgroups tests suggestion on this link http://community.jboss.org/wiki/RunTwoInstanceOfJBossInFedora and during this test no unwanted messages were exchanges. During the test I was able to control wether any messages were transferred by specifying either matching multicast addresses (mcast_addr) to send messages successfully, or use different addresses to prevent any messages from being received.
You need to read that carefully, as the priority on the page is given to AS 5, which has different behavior from AS 4.x.
The problem you are seeing sounds like http://community.jboss.org/docs/DOC-9469 -- much of the content on the first link I gave is related to avoiding this problem.
It's odd to me that you were able to isolate clusters on a Linux machine simply via the multicast address running the McastSender(Receiver)Test with the JGroups version that ships with AS 4.2.3.
If you upgrade the JGroups version to 2.4.5 or later (might as well use the latest 2.4.x -- 2.4.7) then this problem should go away.