I may be wrong, but you're not liable for anything your customers do to themselves. If they trash their laptops by downloading garbage, it's their problem, not yours. If some ambulance chaser wants to drag you to court claiming that you are responsible for his actions, I don't think it will stick. Methinks you're mixing this with the situation where you are resonsible for employees surfing habits. That's different.
The real danger these days is having someone sue you for totally fabricated grounds, without much basis, and then offer to settle. It's often cheaper to pay the extortionist than to defend yourself in court. There's little you can do in your wireless setup to prevent that from happening.
If you absolutely insist on playing censor in order to protect yourself from an imaginary liability situation, methinks it would be best if you first contact an attorney to see what your exposure might be. My guess(tm) is that you have no responsibility as long as you do NOT touch the content, which means you play common carrier instead of censor. One thing for sure... you're going to have complaints from customers if you filter.
On the other foot, if you insist on filtering, there are various web proxy filtering services, usually run by religious groups, that will sanitize your customers internet access. I don't have any experience with these. Some URL's found with Google under "web content filtering service":
(lots more...) However, these filter only by content on port 80 and do nothing for peer to peer. The real problem with peer to peer is that they will hog ALL your bandwidth which is guaranteed to cause complaints. Some type of traffic monitoring followed by blocking the users MAC address is probably the best way to deal with that.
You probably need some kind of per-user bandwidth cap, then you don't have to do traffic analysis, you just limit them to (say) a megabyte per cup of coffee. I dunno if any hotspot-in-a-box products do that, but some of the services probably do.
Yeah, that will also work. What I meant by traffic monitoring is tracking how much a user downloads by MAC address. If it exceeds some limit, such as your xx megabytes per coffee cup (do refills count??) or xx % of available bandwidth, just throttle their connection with QoS or just ban their connection. No need to "detect" file sharing. Just measure their overall use.
If starbucks et all is next door,, do you want their business?? are you also a coffee shop??
if so, try one of the airepoch hotspots with a 7 dbi vertical and a DSL connection. Give it out Free, limit all 'inter wirelesss' connections so people can't play games between each other on your hotspot, and give it away, but advertize on the sides of the browser.
You will have about $500.00 invested with a router and DSL modem, antenna, hotspot, etc. advertise on the sides of the browser and you will have a hotspot that 'runs itself.
Then if you really want to you can sell service by the hour, week, month, etc