How to stop getting junk mail?

I keep getting junk mail! What can I do about it? What bothers me is that they are all for dating sites that I am supposedly signed up for. I am not signed up for any dating site! However, I do know a friend that is signed up for dating sites. Is it possible that one of those sites got a hold of his contacts? Or is it possible that his computer may be cracked and someone got his contacts that way? It would not surprise me. He is very computer illiterate. What can I do to get rid of all of this junk mail?