During a day's play at Skirmish Mansfield some arse dug up a piece of camo net which had been partly trodden into the dirt and insisted that it was now his. I tried my best to make him understand the concept of "not yours; not your land: thieving bastard" but he wasn't having any of it. His brain just didn't seem able to compute that simply digging half of something out of the ground did not somehow convey ownership of said item - he's probably a frustrated metal detectorist.
When the site isn't in use they have multiple motion sensors as well as fixed alarms, all of which text staff, and video surveillance, so problems have been few and far between. However their is another site a few miles away whose owners used to do business with Skirmish's people and are now disgruntled after they went off in a huff and subsequently died on their arse business-wise... they are believed to be behind the theft of signs on the main road away from the security zone.
TBH, as others have said, by their very nature sites are easy targets, so the way forward is either to build fixtures out of non-flammable materials and/or use proper modern security, which is a shame for small time operators just starting up. There are enough things to worry about without the prospect of people deliberately fucking you over...