DONDE EN condición de no aceptar valor de cadena

Puedes hacerlo usando una colección:

CREATE TYPE VARCHAR2s_Table IS TABLE OF VARCHAR2(100);
/

Luego ingrese sus datos así:

SELECT *
FROM   table_name
WHERE  user_data MEMBER OF VARCHAR2s_Table( 'abc123','xyz456','pqr789' );

Alternativamente :

Puede crear una función para dividir los datos y generar la colección:

CREATE TYPE VARCHAR2_TABLE AS TABLE OF VARCHAR2(4000);
/

CREATE OR REPLACE FUNCTION split_String(
  i_str    IN  VARCHAR2,
  i_delim  IN  VARCHAR2 DEFAULT ','
) RETURN VARCHAR2_TABLE DETERMINISTIC
AS
  p_result       VARCHAR2_TABLE := VARCHAR2_TABLE();
  p_start        NUMBER(5) := 1;
  p_end          NUMBER(5);
  c_len CONSTANT NUMBER(5) := LENGTH( i_str );
  c_ld  CONSTANT NUMBER(5) := LENGTH( i_delim );
BEGIN
  IF c_len > 0 THEN
    p_end := INSTR( i_str, i_delim, p_start );
    WHILE p_end > 0 LOOP
      p_result.EXTEND;
      p_result( p_result.COUNT ) := SUBSTR( i_str, p_start, p_end - p_start );
      p_start := p_end + c_ld;
      p_end := INSTR( i_str, i_delim, p_start );
    END LOOP;
    IF p_start <= c_len + 1 THEN
      p_result.EXTEND;
      p_result( p_result.COUNT ) := SUBSTR( i_str, p_start, c_len - p_start + 1 );
    END IF;
  END IF;
  RETURN p_result;
END;
/

Entonces puedes hacer:

SELECT *
FROM   table_name
WHERE  user_data MEMBER OF split_String( 'abc123,xyz456,pqr789', ',' );

o:

SELECT *
FROM   table_name
WHERE  user_data MEMBER OF split_String( TRIM( '''' FROM '''abc123'',''xyz456'',''pqr789''' ), ''',''' );