The following example demonstrates trapping the Windows WM_SYSCOMMAND message. If the command specifies to minimize or maximize the form, we will beep the PC speaker instead;

Example:
type
  TForm  = class(TForm)
  private
    { Private declarations }
    procedure WMSysCommand(var Msg: TWMSysCommand);
      message WM_SYSCOMMAND;
  public
    { Public declarations }
  end;
var
  Form : TForm ;
implementation
{$R *.DFM}
procedure TForm .WMSysCommand;
begin
  if (Msg.CmdType = SC_MINIMIZE) or
     (Msg.CmdType = SC_MAXIMIZE) then
    MessageBeep(0) else
  inherited;
end;