Void trapping them? Killing their pet? Update suppression greifing? Breaking all the torches near their house? Just straight up stealing all their loot? Infinite bell ringing noises?
Accidently punching your friend; "Accidently" punching your friend; Trapping your friend in a pit trap; Trapping your friend in a pit trap (With Lava); Trapping your friend in a pit trap (With TNT); Giving your friend a one-way ticket to the void; Waxing your friend's copper so it doesn't oxidize; Putting string over your friend's crops so they don't grow higher; Trapping your friend in the nether; "Accidently" activating the trap in a desert pyramid so your friend can't get the loot; Tricking your friend into thinking there's a TNT trap in their base; Throwing your friend's favorite sword into cactus; Putting, like, 10 nether portals under your friend's house so they get a ton of annoying noises; Planting bamboo all around your friend's house so they get ultra lag; Stealing the enderdragon kill from your friend; Punching your friend when he was super close to mining obsidian; Putting, like, 10 end crystals inside your friend's house; Blowing up your friend's base that took them 5 hours to build; Trapping your friend in one of those "unescapable traps"; Siccing the warden on your friend; Killing your friend's pet; Telling your friend to meet you at the coordinates 10000, 100, 10000 and then when they are there saying "Oh did I not say negative?"