Simpliest solution is to use a timer

<code>
procedure Tnewform2.FormShow(Sender: TObject);
begin
   tmr1.enabled := false;
   tmr1.interval := 1;
   showmessage('not allowed to open this form');
   tmr1.Enabled := True;
end;

procedure Tnewform2.tmr1Timer(Sender: TObject);
begin
   tmr1.Enabled := false;
   close;
end;
</code>

Note: just make sure tmr1 is disabled during create to avoid using resources