Can I restrict ios to only one WiFi connection/SSID? We have about 20 iPads for our business. We would like to restrict these devices to only be able to access our protected WiFi here and the devices from connecting to others. Is there an app, setting, etc that will do this?
This answer is short and simple: No, you cannot do this.

You can simply add a chain to the iPad so it cannot leave the building if that might help you, but any other way of restricting access to (free) WiFi Access isn't possible.