[opel431]

Можно так (Delphi 7).

Ставим BorderStyle -> bsSingle

private
procedure WMSysCommand(var Msg: TWMSysCommand); message WM_SYSCOMMAND;


procedure TForm1.WMSysCommand(var Msg: TWMSysCommand);
begin
if ((Msg.CmdType and $FFF0) = SC_SIZE) then
begin
Msg.Result := 0;
Exit;
end;
inherited;
end;