There was a similar discussion about domain names a few weeks ago. It’s one thing to accidentally register a domain that is similar to someone else’s. In these instances, chances are that each domain will have different content, links, keywords, etc.
But to deliberately register a similar domain, and then to go and spam a bunch of blogs with their unoriginal domain…. that’s a double no-no, in my opinion. I would be upset too if I was in your position.
You said the person wrote back to you. Did he explain why he did this? Did he apologize? I’d keep an eye on that URL for a long time to make sure he doesn’t switch it back.