A couple Ideas to add to those already suggested...

by webserf - 3/23/12 7:12 PM

In Reply to: Rather than trying to close ports to block undesired usage, by Flirkann

Turn off (schedule) the wireless radio at night so that people cannot tap in when the place is closed.
Remind your customers, with signs and other means that the connection is NOT meant for extensive video streaming or large file downloads.
Change your pass phrase every day, (this will become a PITA most likely).
Consider charging a nominal fee OR a minimum purchase for use of the "Free" internet.

Look into the Ad-served "Free" wifi service similar to those offered at McDonalds.

Most importantly: Look into an unlimited data plan so that you can rest easy regarding anyone who abuses the service. IF you need to pay more for an unlimited plan, you can make adjustments to your prices to cover the costs.