EAP-TLS is the best solution.
You can deploy a Microsoft CA server (included with Windows Server), create a group policy to automatically deploy certificates to AD members, and configure the WiFi to use those certificates.
Getting the certificates onto mobile devices using this solution is difficult; you need an MDM. You could use a separate SSID for the mobile devices that only provides Internet access.
If you *really* want to stick with PEAP, you could create an AD group policy that allows only "machine" authentication to the SSID. Then tell NPS to only allow "Domain Computers".
You could authenticate the mobile devices onto a separate SSID that only provides Internet access, which allows AD username and password.
You could also use NPS to push a VLAN tag. "Domain Computers" go onto one [internal] VLAN, "Domain Users" go into another VLAN (with Internet only access).