How to Stop iPhones from Connecting to a DHCP

Hello,
We have a quest wireless account that is open and doesn’t require a password. This is nice when people with their personal labtop and need internet access. Problem with it is their are alot of iPhones that keep connecting and filling up my DHCP. I know I could block based on MAC address but anyone with a Apple computer would be blocked aswell. Is there a way to block based on Name of device? Most users don’t change the default name of their iPhone, or anyway else?

Thanks