They'd have fun trying to block off Harwoods: it's a huge open shaft. It is the bane of the Search and Rescuers though, as it is a fairly regular haunt of muppets. In some of the local climbing guides it suggests that it's a good trip on a wet day when the crags are too wet. A few years ago a mate of mine got called over to a rescue there. Seems that a party of climbers had started to descend and a girl in the group had gone down the entrance shaft (180m and very spacious), dropped maybe 50m or so, crapped herself, pendulumed across and grabbed a shrub, managed to unclip her figure 8 from the rope, let go of it, and became very well acquainted with her stupidity! Dr Mike Brewer (same guy had an accident in the area a while ago) comes along a few hours later, descends, realises that there's not a lot he can do, swings in, clips onto her, and swings out again. The panicky tart probably needed to be smacked around a bit!
Not a lot that can be done to effectively control access other than to warn. Keeps the rescuers on their toes, anyway....
Saying that, it's a good trip and pretty impressive. Always thought it was a shame that the Kiwi cavers didn't spot the potential for caver recruitment with a place like that on the doorstep. Ideal place for a big-boys winch meet!