Tell the healer not to heal him and let him die. He will learn.
This is even easier when the healer so happens to be your wife.
Just a week or so ago, I ran an Underbog PuG without my wife as my healer. We killed the last boss and some tanking shoulders dropped (we are talking stamina and defense, with very little strength) and the Death Knight in the group rolled need and took them, even though they were an upgrade to my existing tank shoulders. The rest of the group and I told him he should allow the person that would get the most use out of the shoulders to have them since he got all the loot prior to the last boss and his original shoulders were better for DPS anyways, but he instead dropped group.
An hour later my wife was finally home and we both decided to queue for a dungeon. We got Crypts and that same Death Knight appeared as one of the DPS. I knew it was going to be time to have a little fun, so I told my wife not to buff him or heal him, and I stood around for a minute. He grew impatient so I told him "Well you got some nice tanking shoulders, go tank." and so he did, and died. My wife refused to ress him, and when asked why, she said "because I am with the tank, we don't like ninjas". He was forced to run back, my wife and I laughing.
The rest of the time he stood behind, making sure not to get aggro since I was not pulling off him and my wife was not healing him. Once we got to the first boss I decided I was not going to let him has a chance to get some loot, so I initiated a vote kick and he was gone when all of us voted him out.
Yes, I can be vindictive sometimes. I don't like people taking loot they don't need over someone that does need it, whether it is me getting the shaft or someone else. I literally stopped a group once to make a druid give a staff to a priest because he already had the heirloom staff.