I once parked my bike in the town centre and when I came back two Eastern European birds were sat on it taking pictures of themselves. I thought fair enough they're not doing any harm. They were unaware I was watching them, so once they I thought they had had enough time I set the alarm off on my bike with my remote. They proper shit themselves it was proper funny.
On a more serious note, I am always having people (kids especially) messing with my bike. Once I came out of the supermarket to find two kids messing with my bike. The worst thing was their mother was watching them and in a sense encouraging them. As I approached my bike they noticed me (although they did not acknowledge me) and continued to mess with my bike. So I asked the kids politely to not touch. Then the mother took offence and told me that they are only kids. I was really pissed off, I could tell she was a proper chavvy Jeremy Kyle wannabe slag. I couldn't belive that she was making me out to be in the wrong. I then politely explained to her that bikes are easy to damage but expensive to repair and that her kids might accidentally break something. Even though she went away then, she still did not understand my point of view. Stupid bitch, I dread to think what sort of adults her kids are going to become having her as a role model.
I work with 16-25 year olds and some of them are problem kids who are from social services. One lad in particular was always messing with my bike. He burnt the cotton coverings on my chains, ripped my bike cover, was continuously setting the alarm off to annoy me, the worst thing he did was take a rubbery/plasticky cover from a cluster of wires near my headlight and throw it away, he then messed with the wires. Luckily he didn't cause any damage and I made a replacement cover.
I wanted to strangle the little cunt, but obviously I couldn't.