Displaying Error in user exit and stay on the tcode in edit mode

Hi, 

I want to throw a error in EXIT_SAPLCNAU_004  but stay in the tcode in edit mode withought saving. how do i achive this?

when user is trying to do NWA deletion flag set (DLFL) i want to stop user based on condtion.  i have below line of code which is not working as expected.

IF SAP_ACTVT = '02'. "check if user is changing

     select single * from AFVC into t_AFVC where OBJNR = SAP_AFVGD-objnr.
     if sy-subrc = 0.
          if t_AFVC-PRKST NE 0.  "cost is not zero
               SAP_X_ACTVT = ' '.
               MESSAGE 'NWA can not be deleted. The Amount is not zero' TYPE 'E'.
          else.
               select single * from AFVV into t_AFVV where AUFPL = t_AFVC-AUFPL and APLZL = t_AFVC-APLZL.
                    if sy-subrc = 0.
                         if t_AFVV-ARBEI NE 0. "work is not zero.
                              SAP_X_ACTVT = ' '.
                              MESSAGE 'NWA can not be deleted. The Work is not zero' TYPE 'E'.
                         endif.
                    endif.
          endif.
     endif.
ENDIF