Criminalise Your Customers?
Tara pointed me to this story about a Dutch coffee-shop chain that
By continuously changing the names of their store networks to such things as OrderAnotherCoffeeAlready, BuyCoffeeForCuteGirlOverThere?, HaveYouTriedCoffeeCake?, BuyAnotherCupYouCheapskate and BuyaLargeLatterGetBrownieForFree...is able to both promote items as well as guilt patrons into realizing free WiFi really isn't
That's smart as far as it goes. But it's indiscriminately interruptive of all wi-fi users and, more importantly, guilt is not the primary emotion you want to engender in your potential customers.
Far better to focus on the wittier messages rather than the hard sell. Far better to seek to make the miscreants (not to mention all those freer-spending wi-fi users) feel part of the community and invested in the fate of the business. Far better in the long run and far more likely to cause them to return.
Addendum: In line with this downtime interaction spotted by Iain, they could also use their interruptions to seek positive opinions or poll their miscreants about why they're not buying coffee.