You can use a regular expression to do that - depending on how many chars you want to keep and how many you want to get rid of.

Here's a nice discussion on some various ways to do it

https://community.oracle.com/thread/595376

or if you want dont like regex, you could convert it to ascii str and then rip out the non-printing elements, eg

declare
  x varchar2(100) := 'Hello Ä Ê ÍThere';
  y varchar2(200) := asciistr(x);
  res varchar2(100); 
begin
  while instr(y,'\') > 0 loop
     res := res || substr(y,1,instr(y,'\')-1);
     y := substr(y,instr(y,'\')+5);
  end loop;
  dbms_output.put_line(res||y);
end;
/