There is nothing you can do to stop it. Port scans occur by the 1000's all the time. People are looking for places to attack, research projects scan for data, ISP's scan for customer connection and usage. That's what we have firewalls for. Being connected to the net means being connected to 100's of other places at the same time. We all just hope we can trust most of them. You are probably connected to 50 or so places right now. Try
Current Ports and enable all the connection views to see who is connected to you right now.
That particular domain is in Annapolis, MD. Maybe the military or NSA.