I recently read the story of the coffee shop that's shutting down their free wifi on weekends because it mostly gets them moochers who, far worse than simply not buying anything, sit and stare at computers and don't talk to anybody. They found that when they shut down the free network, they not only got people to buy more coffee, the place was also more social.
So while there are a variety of solutions to sell or control access to a network, such as printing tokens that give a period of access on every receipt, or selling the access as they do at Starbucks, here's another idea -- intermittent access.
In such a system the access point lets you on for a modest amount of time. Enough for a quick web search or two, a checking of your e-mail or even a modest phone call. Then it denies you access. It doesn't have to deny it for long, perhaps just 5 minutes before you can get on again. No authentication, though during the period of denied access, it may redirect all web requests to a page that explains the situation, and optionally offers continuous access for money.
Though that's not the main goal. The main goal is to create an atmosphere where you're coming to the shop to do other than stare at your computer, but in which you can use it on occasion to get your fix.
Who knows, if the sale option for continuous access was popular, it might even make more money than an always charging system. Of course, fancy users could change their MAC address to get around it -- but if they're going to go that far, let them. Most won't.Another good feature of this system is that for most people, it doesn not have any authentication scheme. Hotspots that force you to log in are nothing but trouble. They cause user and support headaches, and they block access from non-browser devices such as VoIP phones and who knows what else people want to invent. The short term acess can work without requiring the browser login session. Indeed, if you wanted to be smart, it could look at the application in question and allow extended time for some things that might need them.
Of course you would need to be on a browser to see the "take a break, talk to your neighbours" screen after your time is up.