New more prominent signs will be a good start, most people are ignorant of the fact that payment is required, and therefore ignorant of the site rules, the current sign on the gate is highly missable.
An article in Skywings will also do a lot of good, i believe Nick is on the case.
Some people are just selfish arseholes of course, and you won't stop them doing as they please, but thats true in all walks of life.