Don't respond is great advice.
Also, if you are a snooper, stop! You don't need to know what's going on in his life.
If you still have mutual friends, let them know you do not want to know about him. He is not a topic of conversation/gossip.
If you share any joint accounts/bills, get them separated and get his name off so you don't have to see it.
Same goes for mail. If his mail is still coming to your place, forward it to wherever he is so you don't have to see his name when you pick up the mail.
Get rid of his pictures. I have one picture of ex in my house and it is one with our children when they were younger (they are grown now) and that is there for my children.
Basically, get rid of any physical evidence that reminds you of him.
The harder part is to stop the mental longing. It's not easy, and takes time. When I found myself getting nostalgic, I said STOP! (out loud if I was alone), and immediately turned my mind to something else (like a vacation I was planning, or a project I was working, or a fantasy guy). I had several "something else" stand-bys at all times so that I wouldn't be searching around for something else to focus on.
Over time, you will find that he will take up less and less of your head space.