Connected to:
Oracle9i Enterprise Edition Release 9.2.0.6.0 - 64bit Production
With the Partitioning option
JServer Release 9.2.0.6.0 - Production

SQL> -- make sure we can store UTF8
SQL> select parameter, value, banner
  2    from v$nls_parameters a, v$version
  3    where parameter like 'NLS_CHARACTERSET%'
  4    and banner like '%Enterprise%';

PARAMETER
----------------------------------------------------------------
VALUE
----------------------------------------------------------------
BANNER
----------------------------------------------------------------
NLS_CHARACTERSET
AL32UTF8
Oracle9i Enterprise Edition Release 9.2.0.6.0 - 64bit Production

SQL> CREATE OR REPLACE FUNCTION ascii_only (p_txt IN VARCHAR2)
  2     RETURN VARCHAR2
  3  IS
  4     v_tmp     VARCHAR2 (32767);
  5     v_clean   VARCHAR2 (32767);
  6     v_char    VARCHAR2 (3 BYTE);
  7  BEGIN
  8     FOR i IN 1 .. LENGTH (p_txt) LOOP
  9        v_char := SUBSTR (p_txt, i, 1);
 10
 11        -- choose your range of valid characters
 12        -- Ascii only looks at first BYTE of character
 13        -- (note sure if this is a reliable way to detect UTF8 multi-byte characters)
 14        -- (but it seems to work ok on my 10000 test records)
 15        IF    (ASCII (v_char) BETWEEN 32 AND 127)
 16           OR (ASCII (v_char) IN (9, 10, 13)) THEN
 17           v_clean := v_clean || v_char;
 18        END IF;
 19     END LOOP;
 20
 21     IF LENGTH (v_clean) != LENGTH (p_txt) THEN
 22        DBMS_OUTPUT.put_line ('removed '||TO_CHAR(LENGTH(p_txt) - LENGTH(v_clean))||' characters');
 23     END IF;
 24
 25     RETURN v_clean;
 26  END;
 27  /

Function created.

SQL>
SQL>
SQL> create table utf8_data(col1 varchar2(20));

Table created.

SQL>
SQL> -- http://www.unicode.org/Public/3.0-Update/NamesList-3.0.0.txt
SQL> -- Euro is U+20AC, Small Greek Gamma U+03B3
SQL> insert into utf8_data values ('hello '||unistr('\20AC')||unistr('\03B3')||chr(10)||'world');

1 row created.

SQL>
SQL> set serveroutput on size 1000000
SQL>
SQL> select ascii_only(col1) from utf8_data;

ASCII_ONLY(COL1)
---------------------------------------
hello
world


SQL>
SQL> begin
  2   DBMS_OUTPUT.put_line ( 'just flushing buffer...');
  3  end;
  4  /
removed 2 characters
just flushing buffer...

PL/SQL procedure successfully completed.