You can create a firewall policy to deny all traffic to destination port UDP 443 coming from the subnets where those iPhones connect from. That solution anyway would block traffic to port UDP 443 for all devices on that specific subnets, including iPhones.
My understanding is that you want to force the iPhones to avoid using the destination port UDP 443. If you block the traffic on destination port UDP 443, the application cannot use that port, so you achieve your goal. If the application is not able to function properly without using port UDP 443, that is an application problem I guess.