From the article:

The Teufelsrad, or Devil’s Wheel, which rapidly spins around while people try to sit on it for as long as they can

So I imagine it would be the same as just doing away with it entirely to install restraints on the ride. That said, why not just cordon off a space around the ride and only permit those who are participating to get close? You could put down padding to prevent injuries when people fall off, and put up a low wall that allows folks to watch their friends as they ride it, yet is high enough to obstruct the view of those who have been through off it. Since it's an annual event, it seems like the costs would quickly prove better than handing out free cycling shorts every year, versus just making the necessary structures once and installing and removing them once a year. Would also more thoroughly block would-be perverts.