Makes me so angry to hear about such vandalism. I'm aware I might be starting to sound like a broken record, but please report it to the police. I *ahem* wrote a handy article about the best way to go about it here:
Thanks J, I had your article in mind when I saw the spray paint. The difficulty there is it's not ongoing dumping or damage by the landowner, it's probably just some scrote who won't be back there. But I shall report it to the local archaeos. There was also evidence of a small (camp)fire up at the cairn on the highest point of the hill.