What do the other players say about the one player's behavior? I'd expect they are not thrilled. Maybe this problem can be solved within the group without your intervention. If the other players are OK with the behavior, but you aren't, then you should use the tools that are at your disposal, which is control over the world and the game setting.
Here is a suggestion how you could teach that player a lesson:
Introduce the concept of Karma
Karma is the Buddhists' belief that your actions in this life defines how well you will start your next life. Do good and you will be better, do bad and you will be worse. Obviously there is no next life here, so you introduce instant Karma here, which means that if a character behaves well, he will be rewarded with better outcomes. Or in that case that selfish actions will be eventually punished. Of course instant punishement is boring and obvious. Here is a more indirect suggestion:
Phase 1: The trap
The character in question finds an item in a situation you have described and loots it. The kind of item is at your discretion, but it should make the character significantly more powerful. It should be also obvious that the item is powerful, so that your player is guaranteed to loot it. Unbeknownst to the player though, this item carries a curse.
Phase 2: The investment
Shortly after acquiring the item, the character discovers that he can make the item even more powerful by "investing" in it. This can be a material (gold, gems, other items) or immaterial (abilities, points etc.) investment of items that he already has into the new shiny item that makes it more powerful at the expense of the invested item. He loses something to gain power through his looted item. A variation on this is that from a mixed loot that is found somewhere he gets "extensions" to his item and leaves other (though still valuable) loot to the other players.
The character has now gained an even greater attachment to the item, which makes it more valuable to him.
Phase 3: Group dynamics
EDIT: As it has been pointed out in the comments, this is a risky option since it is somewhat pitting the players against each other. Actually, this step is entirely optional. You know your players best and it is up to you to decide how they would take it. If you're unsure, just skip this step.
The curse on the item is starting to slowly show an effect. It turns out that the item gives its bearer some benefits which come at the expense of the other members of the group. For example:
- Things get harder to achieve for the other group members, while they get easier for the bearer of the item.
- Some NPCs appear that have an interest in the item. Since the item makes the summoner untouchable for them, they are harrassing the other group members.
- Some NPCs appear that see the summoner as the leader of the group because he holds the item. The other group members are seen as his subordinates, servants or even slaves. He is given the best food, a good place to rest while the others are not treated the same. When the group is asked for council, the other members' opinion is obviously ignored, while the summoner's opinion is held in the highest value, regardless of what he says.
This enhances the effect that the character's looting behavior has on the group. Most likely the other characters will start to resent him.
Phase 4: The comeuppance
When you feel there is enough tension in the group (it still is a game, so you don't want a fully fledged conflict), you can finally close the trap on the player.
The group will find itself in a situation where the only escape from certain death is a door that can be only unlocked with the looted item. During this process the item is inevitably and completely destroyed. All the investments the character has made in it are lost forever. If you fear the character will rather let everyone go down with him than giving up his loot, just put him in the situation and leave the others out of it.
You can enhance the effect by giving the other players an extra benefit at this point while the former holder of the item gets nothing.
That will set an example to all the players that you won't interfere with their actions, but also that their actions will have consequences.