Wow, that's scary. I never had anything that bad happen, but I kept my horse at a campground/RV park I was managing for a few months, and I would daily find people messing with her. She had a portable corral that I would move around to keep her on fresh grass, which had large signs posted all over saying it was electric, do not open it, leave the horse alone, do not feed/pet without owner present, etc. I would find that people had dumped the leftovers from their lunches into her corral, kids ducking through the fence and running under her legs, and she was a fairly aggressive horse towards strangers, especially when food was involved, so it made for one very nerve-wracking few months. I was afraid of someone accidentally letting her out because the property was not fenced (hence the portable corral) and backed up to a major highway which had semi trucks running up and down it all the time at high speeds! I had no problem if people would come to the office and ask me if they could pet or feed the horse. I just wanted to be there for it, because I would know how she would react and be able to control the situation.
Lock everything that locks, and report the incident. Put up no trespassing signs. Maybe put up Beware of Dog signs. Or make a sign saying Do not Feed or Pet horses. "Danger High Voltage" might be nice on your electric fencing. If multiple properties around you are having this problem, maybe you could ask the police in your area to keep a closer watch on the area? When I had an apartment of mine broken into, they were more than happy to have an officer make a circle of my block at least a couple of times a day looking for suspicious activity. A security camera would be good too if you can afford it. Or even a fake security camera located in an obvious spot so they think they are being taped.