solved.. I just learned we need to handle the bulk delete operations
by ourselves, JPA do not cascade the delete to the relationships....
@Override
public void delete(long id) throws IllegalStateException,
IllegalArgumentException, TransactionRequiredException,
PersistenceException {
Query query = manager
.createNamedQuery(FpCertificate.BULK_DELETE_CERTIFICATES);
query.setParameter(FpCertificate.USER_ID_PARAM, new Long(id));
query.executeUpdate();
FpUser user = manager.find(FpUser.class, new Long(id));
manager.remove(user);
}