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;