It does need reporting, the police have been known to actually confiscate and destroy bikes around here. It is a problem in the quarry, they come in off Rooley Moor (which they also shouldn't be anywhere near). Given the amount of money and effort that has recently gone into Lee Quarry these tossers need shooting (I don't understand why they think they have the right to come in and destroy someone elses legitmate work and endanger those people who have a legitmate right to be there). They do have facilities locally (as pointed out above). They are incredibly dangerous, I've had them go past me at speed a number of times and get a face full of gravel as they pass. They've wrecked some of the bridleways locally.
Not sure how they're going to stop them though, the top of the quarry isn't fenced and neither is Cragg Quarry. There has been enough erosion in Lee Quarry in places just from MTBs, a few MXs will tear it up completely.
I don't have any sympathy for them either, we are restricted as to where we can (legally) ride, they're even more restricted. Thing is if everyone had complete freedom of access the 4 x 4 drivers and MXs would effectively drive everybody else off from some areas.