I've given this a bit thought and can't think why you would want to prevent the deletion of a stateful session bean. A client creates the session and so it would be a bit strange if there was code to prevent it being deleted in the bean class. As the instance can only be used to service that one client which it was created by and no others.
A container might want to prevent the deletion say, if the bean was using BMT and the client was in the middle of a transaction. That's the only reason I could think of that a stateful session bean might not be deleted.