Delphi : how to completely remove String from the memory

Var
S1:String;
S2:String;
begin
   S1:='Sensitive Data';
   S2:=Crypt(S1,'encryption key');
   S1:='';
   FreeAndNil(S1);
end;

now when i search on my process memory using programs like "WinHex" i can easly find the un-crypted String ! even i tried to make new form to encrypt this string then unload the form but it still exist

is there any way to completely remove it

thanks in advance