Автор: http://www.swissdelphicenter.ch

function UnCheckIndex(FileDbf: string): Boolean;
 var
   Dbf: file;
   Car: Char;
 begin
   Result := T;
   AssignFile(Dbf, FileDbf);
   Car := #0;
   {$I-}
   Reset(Dbf, 1);
   if not ErrorIO(FileDbf, IoResult) then
    begin
     Seek(Dbf, 28);
     {Flag's position}
     if not ErrorIO(FileDbf, IoResult) then
       BlockWrite(Dbf, Car, 1, Num_R)
     else
       Result := F;
     CloseFile(Dbf);
     if ErrorIO(FileDbf, IoResult) then
       Result := F;
   end
   else
     Result := F;
   {$I+}
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   if UnCheckIndex('MyBase.dbf') then
     ShowMessage('Flag removed');
 end;