I do not think you can have read-only for session bean's methods.
Look at the @ejb.transaction (0..1) tag for the bean methods. You can use type="Supports" or "NotSupported".
Supports: If you are not sure if this should participate in the transaction.
NotSupported: If you are sure that you do not want to rollback based upon exceptions from this method.
The dtd for the jboss.xml http://www.jboss.org/j2ee/dtd/jboss_3_0.dtd
supports the the idea of the read-only methods on session beans and entity beans.
This is a jboss entension and not part of the ejb spec.