Cursed items.
Maybe a magnetized piece of armor, which metal weapons stick to (DC 25 Strength check to pry weapon off). The first time he can't attack in combat, he'll start to learn.
If she's a spell caster, how about a ring of feeblemindedness? Appears to be a ring of X (invisibility, flying, whatever). And maybe really is, for about 3 uses. Once it's on and the player tries to activate it for the 4th time, the true nature is revealed.
To be super mean, cursed items can only be removed by Wish, Limited Wish, or Miracle. To be regular mean, they can only be removed by Remove Curse.
There's always the old first edition standards like "Helm of Opposing Alignment" and "Belt of Opposite Gender" (actually called a "Girdle" I think, but that name is a dead give-away in 3e).
If the character is Good, perhaps a shift to a Neutral alignment might be in order. If the character is a cleric, paladin, or other divine worshipper, this alignment shift could have bad consequences for the PC.
Those are "in game" solutions. Out of game, you might try talking to the player in private. Or talk to the other players. Maybe it's not a problem. But if it is, you have the power to make Greed hurt.