To avoid that instances are build a cluster:
until JBoss 5.1 it is easy you can use '-g <PartitionName> -u <JGroups address w.x.y.z> -m <JGroups mcast port number>'
Autodiscovery from client side you have to add the JNDI-Property jnp.disableDiscovery' => true
This avoid that the client start an mcast to discover any JBoss in your network (the mcast address is always set to a default)