They put a sticker on people's devices and ask them top pretty please not use them. Anyone can still go there and use their phones as much as they like. It's another social advertising campaign by 'Parks and Wildlife Finland', intended to be spread around internet forums and social media. Doesn't cost them anything, it's a clever idea. And some food for thought, you'd never know about it if you weren't online, nor would you ever have heard of this random island. Of which Finland has hundreds of similar ones.