In jbosscmp-jdbc.xml, you can specify an entity tag
which will make the EJB read-only. This should increase performance as it will not call ejbStore(), and should invoke an error if you attempt to call create().
This is mentioned on page 80 of the JBoss.3.0QuickStart.Draft3.pdg. It also delves deaper in JBossCMP.pdf, describing how to make individual attributes read-only (page 18).