This guy is freaking me out

There’s a man on a networking website I signed up to that keeps putting “MINE” and “Leif’s Girl” all over my pictures. I don’t know the guy, and he’s 48!! My boyfriend is getting really cheesed off with it as well, and wants to pretty much tell him where to go, but the guy doing this has already warned him that if he sends him a message saying to get lost, he’ll report it to the website and get his profile deleted. This crazy man also has said to his friends that he’s going out with me, and was very graphic about the things “we do”. He even sent me a video of him being very rude - verging on porn!! I emailed the person that runs the website a month ago when it first started but nothing has been done about it :-( I haven’t accepted his friend requests, but he still contacts me every day. I really want this to stop!!