(This is anonymous because I don't know any other members.)
At school there was a little git four years younger than me who was a fellow boarder. He got on my nerves a lot, just by being himself, and he was very fond of using the computer room in the evening. My best friend at school was paid to set-up the school network we had at the time, so I got him to hack into the git's user account, planted some pornographic pictures on his area, and anonymously reported it to the housemaster. He was suspended for three days, lost his area, and was banned from using the computer room for the remainder of the term; he had nothing fun to do in the evenings.
When I was thirteen, there was a sixteen year old who rode around my area on a bicycle bullying younger lads, some of whom were my friends, into giving him their stuff. I once saw his bicycle outside a corner shop, and I used a multi-tool I always used to carry around to cut his brakes. I don't know how that turned out, as shortly after I moved.