Some random stuff I thought of during an all-nighter. To see more Random stuff, look for 8 ways to annoy Pansy Parkinson by RoyalRandomness99 or 8 ways to annoy Remus Lupin by Modern Marauder -Miss RoseWings.
8 ways to annoy Sirius Black
Tell him no matter how many times he uses that shampoo: that cologne; those hair-straightners; he will never be as sexy as Severus Snape.
Shave his head bald in his sleep.
Put itching powder in his boxers
Transfigure his hair into rat tails.
Put posters around the school stating Sirius Black 3 Buckbeak 4ever.
Every time Sirius cracks a dirty joke or makes a suggestive comment make him pay 5 galleons.
Stick a sign up in the common room saying No dogs allowed on furniture.
Dye his hair fuschia pink.