This is similar to what they do in Airports etc. I guess this is done by making the wifi free and all users once connected coming to a static webpage. Once they login they get connected to internet. i.e., a proxy manages the internet connectivity - nothing needs to be done with the modem. But this makes the wifi connectivity non-encrypted - not sure if any other solution for encryption exists.
Another easier way is to provide wifi password to customers and change it daily or at periodic intervals. I'm not sure if any modem supports multiple password.