I have a common email address name too and get tons of email meant for others. If I can tell it's a friend to friend email, I'll reply back saying they have the wrong address. If it's a business email, I won't reply back if it has that notice on the bottom saying "this email is confidential and property of Blah Blah Company and if this message is mis-delivered you must notify us of the mistake and delete the message."
I figure that threats shouldn't make up for stupidity and that if one of their employees was stupid enough to think they have my address or to give it out, strongly worded messages shouldn't make up for that. One guy kept emailing for a couple weeks about the status of a project and the importance to get in contact with him. Oh well.
Oh, and this has happened to me with Paypal. I was nice and emailed the guy after I canceled the transaction. He didn't reply back though.