It is not possible to firewall MAC-addresses of the PPTP connections.
Moreover, why you should care about MAC-address for PPTP client. PPTP client is connecting from the remote location, when MAC-address is not even visible (for PPTP server).
PPTP has nothing to do with MAC-address. For example PPTP client is connecting from another city or country, why should PPTP server care about MAC-address ? Why do you have such requirement ?
IP address should be enough.