Damn you, jayjay! I was gonna post that!
In all seriousness, I’ve had this happen to me a few times. The first time, the message was from someone I knew who simply checked my name by accident. NBD; I just e-mailed her to tell her I wasn’t the person she wanted, she apologized, and that was that.
The second and third times were from some apartment management company wanting to know where my rent check was. The first time they e-mailed me, I politely responded that I wasn’t the person they were looking for, and that they might want to double-check their records to make sure they had the correct e-mail address. The sender thanked me warmly and I didn’t hear back from them until a few months later, when I got another “reminder” about a late rent check. I wrote back to tell them that I wasn’t this person, and was told (rather snarkily, I might add) that this was the e-mail they had in their files, and BTW, where was the rent?! I just ignored this message, got cc’d on a message that “my” rent check had shown up, and e-mails from that company just go in the trash.
Which is my suggestion here. Sometimes, virtue is its own punishment. If the idiot keeps e-mailing you, just filter them into the trash. It’s his problem that he can’t send stuff to the right person, not yours.