if flag_is_on then do;
  communicate message password is invalid; ** THIS IS OPTIONAL.  YOU COULD JUST KEEP THE NOTIFICATION BELOW AND REMOVE THIS ONE. OTHERWISE YOU WILL RECEIVE MULTIPLE NOTIFICATIONS;
  endsas; ** QUITS SAS. NOTHING ELSE IS RUN;
end;
else do;
  flag_is_on = 1;
  run the rest of your program including login attempt;

  if program_ran_successfully then do; ** POSSIBLY USE SYSERR AND SYSMSG TO DETERMINE THIS?;
    flag_is_on = 0;  ** TURN FLAG OFF AGAIN;
  end;
  else do;
    communicate message password is invalid;
    ** NOTE THAT WE LEAVE THE FLAG TURNED ON HERE SO THAT NEXT TIME THE PROGRAM IS LAUNCHED THE FLAG WILL STILL BE ON AND THE PROGRAM WILL NOT RUN.;
  end;
end;