SQL> with
  2    data as (select 'aéñàueoüûç' val from dual),
  3    forbidden_chars as (
  4      select substr(replace(sys_connect_by_path(chr(127+level),' '),' ',''),2) chars
  5      from dual
  6      where level = 128
  7      connect by level <= 128
  8    )
  9  select val, translate(val,'a'||chars,'a') new_val
 10  from data, forbidden_chars
 11  /
VAL        NEW_VAL
---------- ----------
aéñàueoüûç aueo

1 row selected.