Wow, this really sucks. I’m sorry to hear it. :no:
I think you have been nice, and clearly being nice isn’t working. I with those who said call the police each and every time. I have no respect for a young lady who thinks it’s ok to tear down fencing that holds horses in.
In the US, “no trespassing” signs would be recommended. If the case goes to court, she can’t claim that she didn’t know that she wasn’t welcome or that the pasture was private. I don’t know how the UK rules work though.
When she visits, she can only be walking from so far. What if you were to start from the trails leading from that fenceline and visit the nearest houses. Visit them and be friendly. Maybe you can politely enquire if he/she knows where a young 12-13 yr old girl might live? (without telling too much) Once you know where she lives, life will get much easier for you and the police can be more effective.
I also like the ideas of doing something to the outside fenceline that makes it unplesant to handle or harder to take apart. How about just a deep moat with alligators?
Do take notes of dates/times every single time you see her or see something amiss. This will help in prosecution, and it’ll also help you if someone gets hurt & you have to defend yourself.