Blocking wifi from escaping without blocking cell phone can not be done, however there are other options for keeping people from unauthorized access to the WiFi.
What you will need to do is set up a captive portal. After the user connects to your wireless the user will not be able to connect to anything other than the server's login page until they type in a username and password.
Once you have individual logins you can start logging and tracking how often and how long people are logged in. If you see someone is connected for 24 Hours you know they are stealing your WiFi and you can ban them. If you see a login that happens every day but you know that person is not a regular that does not come in every day you can ban that too.
You may be able to replace your WiFi access point's firmware with a replacement firmware, CoovaAP is a example of a firmware that is specifically designed to be used in situations exactly like yours.