How do I get rid of unregistered contacts to stop messaging me on Skype?

This started a few days ago. An unknown contact keeps messaging me saying things about seeing them on webcam and if I have a dating site. It’s obviously a scam. The problem is, I can’t get rid of them.

I right click on their name, but there’s no way to block them. I even go to my “blocked” list and can’t register the user there. They just don’t show up on that list. I can block all of my actual friends, but just not this one person.

How do I go about getting rid of them for good?