I solved it... Finally.
Just place this code into the class of your form.
protected override void WndProc(ref Message m)
{
if (m.Msg == (int)0x84)
m.Result = (IntPtr)(-1);
else
base.WndProc(ref m);
}
This will prevent that form from being active, there for sending all mouse commands 'under' that form; to form 1.