24 hour security guards with big dogs and machine guns. Frowning at you. Especially anyone with long hair who looks like they could do with a wash.

Orrr maybe it'll look like there's no fence, but actually there'll be a system of lasers, so you can get so close, but if you cross the invisible line you'll be zapped - unless you're carrying a special tag that they give you at the visitors centre when you hand over your credit card of course.

Or land mines, the position of which are only apparent if you're wearing the special goggles they give you at the visitors centre etc.

Or, they'll give you something special to wear when you pay your money - a fluorescent druid's cape or something similarly distinctive. And then anyone not wearing one but being near the stones will be shamed by social pressure not to approach any further.