Depending on the usage of your client code, please take a look at the FAQ regarding thread safety of client artifacts: http://community.jboss.org/wiki/JBossWS-FAQ#What_client_artifacts_are_thread_safe
Generally speaking, a widely used strategy is creating a Service just once, then create a pool of proxies to be used by the client.
Regarding the Cannot close output stream message, it's difficult to tell what's happening exactly; generally speaking consider moving to recent JBoss AS / WS versions.