Here's 50. Some are repeats.
#1 - Steal all of their Harry Potter books
#2 - Break their fake wand in half
#3 - Brag that you met Daniel Radcliffe
#4 - Insist that the movies 100 times better than the books
#5 - Claim that you know more about Harry Potter than they do
#6 - Make them wear an EQUUS shirt on their first day of school
#7 - Print out a picture of Evanna L. and then rip it
#8 - Sabotage and rip up their Harry Potter DVD's
#9 - Laugh at the lightning bolt scar
#10 - Sit on their Voldemort doll(action figure, lol)
#11 - Go up to them and say 'THAT MUGGLECAST IS THE WORST PODCAST IN THE WORLD'
#12 - Tell them that you think Rupert Grint is super ugly (i dont, actually i think he's super hot)
#13 - Constantly talk about how much you think that Emma Watson needs to work on her acting skills
#14 - Become a HUGE Harry/Hermione shipper and tell the person that Ginny should die and go to hell
#15 - Call Harry 'HARRY POTHEAD'
#16 - Ask if J.K. Rowling is going to write an 8th Harry Potter book and when they say 'NO' insist that they are wrong and that JKR is
writing an 8th book.
#17 - Wear a sign around your neck that says 'HARRY POTTER PROMOTES WITCHCRAFT'
#18 - Make them cry by saying that Dobby is a stupid ugly elf that deserved to die (8O)
#19 - Close your ears every time they mention 'HARRY POTTER...'
#20 - Say that Hufflepuff sucks
#21 - Videotape them reading the Deathly Hallows and then laugh at all the parts where the cried.
#22 - Hide all of their Harry Potter merchandise
#23 - Make them wear a Harry/Draco shirt on Halloween
#24 - Start supporting Laura Mallory
#25 - Ask them if they've seen OOTP and when they say they have keep telling them that they are lying
#26 - Argue that Michael Gambon was a waaay better Dumbledore than Richard Harris, and if they disagree, say that Micael Gambon is their sweetheart
#27 - Brag that you have tickets to see J.K. Rowling when they dont
#28 - Say that Harry so deserved to die in book 7
#29 - Argue that Harry DID technically die in book 7
#30 - Remind them that Harry Potter is over
#31 - Call Hedwig a stupid little bird that should have died in book 1
#32 - Get a copy of the Deathly Hallows and forge JKR's signature then give to them for their birthday, they'll be thrilled. 2 weeks later tell them the truth.
#33 - When watching movies together keep bugging them by asking what's going on
#34 - Call Daniel a po*n star
#35 - Photoshop their head on a hippogriff costume
#36 - Call Hermione a bitch
#37 - Read their embarassing fanfictions
#38 - suggest they see a psyciatrist because their are obsessed with a fantasy book
#39 - Write a note saying that 'HELENA BONHAM CARTER IS TERRIBLE AT PLAYING BELLATRIX LESTRANGE'
#40 - Call Ron a loser
#41 - Give them bad comments on their youtube videos
#42 - Ask them if they have a dementor problem when the're depressed
#43 - Repeat 'HARRY POTTER AND THE CHAMBER OF POT'over and over
#44 - Write Harry/Hermione fanfiction and send it to them
#45 - Make them wear an 'I HATE HARRY POTTER' t-shirt at a party
#46 - Constantly ask them who dies in book 4 and then mention that Cedric also plays Edward Cullen
#47 - Brag that you've been to King's Cross station
#48 - Constantly mention how Hagrid is always breaking the wizard law
#49 - Annoy them in any way you can
And last but not least #50!!!
#50 - DO ALL OF THE PREVIOUS