You can't do this through the GUI.
Assuming you have a current setup working for all machines (that is NAT is selected in PPTP, proper
You would to run the following command:
/usr/sbin/iptables -t nat -I POSTROUTING 1 -s ! 192.168.10.10 -o ppp0 -j DROP
where 192.168.10.10 is the machine you want to authorise and ppp0 is the PPTP network interface.
This will prevent any traffic due to go through the VPN internet that is not from 192.168.10.10.
If you want to authorise more than one machine, you would have to do something like:
/usr/sbin/iptables -t nat -I POSTROUTING 1 -s 192.168.10.10 -o ppp0 -j MASQUERADE
/usr/sbin/iptables -t nat -I POSTROUTING 2 -s 192.168.10.11 -o ppp0 -j MASQUERADE
/usr/sbin/iptables -t nat -I POSTROUTING 3 -o ppp0 -j DROP
this will only allow the IP 192.168.10.10 and 192.168.10.11
I guess I could add an option in the GUI, but as you're the only one who have made this request …