I used to track down the people who were using my address and set them right. I took a certain delight in contacting people and saying "You don't know me, but you're using my email address and you should stop." In fact, I have an amusing story about it.
Years ago, my home email address was [my first name]@[big isp].net. This was around 10 years ago, and fairly often someone with the same first name as me would sign up and merrily start handing out my email address.
One day I logged in and check my email, and find several email messages clearly meant for someone else. Included in the messages were receipts from online shops, which included the other man's home address, though not his phone number. At first I wasn't going to do anything about it, but then I found a message from a gentleman that this other person had met at a gay bar.
I didn't mind that someone was giving my email address to various vendors online, but now it was a little more personal, so I took action.
Having the person's street address, I tried the phone directory, but found that his home number was unlisted. So I called a friend of mine who worked at a law firm, and asked them if they could do any searches on scary privacy-invading databases and get the man's home phone number. My friend couldn't promise anything, saying "unlisted phone numbers can be pretty hard to get".
A couple hours later my friend called me back. "His number is unlisted, and I couldn't get it. But his wife's number was pretty easy to find."
I called the number and left a message "This message is for [first name]; you've been using my email address, and given the sensitive nature of the email messages I've been getting, you probably don't want to do that."
The misdirected emails stopped immediately.