DesignForExtensionCheck as it exists now seems to flag any public EJB methods. The EJB 3.1 specification (JSR-0003) states several times (sections 4.9.2 and 4.9.6, for example) that EJB classes and methods cannot be final or static. It's not an idle threat, either; Oracle's WebLogic container won't deploy the beans if you violate this.
I'm not really sure how you would fix this, since I don't think you can reliably determine if a class is an EJB or not. Short of removing the check from the default configuration, maybe update the documentation to advise "don't use this check against EJBs?"