Severus Snape
Add a new ingredient to a potion you're supposed to make.
Put a hungry lama that eats hair in his room.
Make him kiss Hagrid.
Destroy his favorite cauldron set.
Give him a set of black robes that that turn lime green and won't come off unless he proposes to Harry Potter,in the great hall.
Lock him in a room with Sirius and Nevile.
Remind him that he was responsible for Lilly's death.
Steal his diary and read the juciest parts aloud at breakfast.
Ask him what kind of baby oil he puts in his hair.
Say"I love you Sevy-Poo" and kiss him at points if you puke in his mouth.