My business hours sign says "WiFi available for customers" on it. We change the password on our WiFi monthly and that works out just fine. The password is only given out to customers who come to the counter to purchase something. So far we've not had a problem with that.
I take a different approach: I welcome free riders.
People add human interest and vitality to what would otherwise be an empty chair and static room or seating area. Non-paying people are of value as scenery that attracts other people, who then become your paying customers. The only exception I make for free riders is when I've a line of paying customers standing around waiting for a seat--then and only then I walk up and say it nicely, "Time to move along, little doggie..."
Why not get rid of WiFi altogether?
Are people coming to your place and hanging out because of your product or your WiFi? Are you a WiFi provider or a coffee place?
And while having people in your shop to fill it out and make it look more "human" is a nice thought, it's not paying your bills or making you a living. I disagree with the assertion that people buried in their laptops is a positive thing to cultivate in your environment.
Get rid of the WiFi!