UPDATE
Ok...I couldn't decide for the longest time what to do. I figured if he gets canned right now, I'll suffer because I rely on him for some things that I don't want to have to assume the responsibility for at the moment. I took the second copy he printed, wrote "So you printed this by accident twice!?!" I'm not that stupid" on it in black marker, and stuck it in his top desk drawer. I also cut off his rights to the color laser.
I feel it was highly inappropriate to print them. We have a somewhat laid back culture here, and no one is going to get fired for receiving a dirty email, but printing it goes to a higher level. Doing anything where another employee can witness it is a problem. And we're not talking some joke email or lewd cartoon, but explicit pornography. He's on an even shorter leash now, and next time he screws up, his ass is grass.