The following demonstrates disabling the buttons and system menu on the form's caption bar. Example:

procedure TForm .Button Click(Sender: TObject);
begin
{Disable}
  Form .BorderIcons := Form .BorderIcons -
                       [biSystemMenu, biMinimize, biMaximize];
end;
procedure TForm .Button2Click(Sender: TObject);
begin
{Enable}
  Form .BorderIcons := Form .BorderIcons +
                      [biSystemMenu, biMinimize, biMaximize];
end;