If I understand this feature request correctly, I would like to share some additional info concerning this. This feature request was already acknowledged by Manuel and can be found on the wishlist (-->
http://m0n0.ch/wall/todo.php) as 'add hidden option to disable automatic firewall rule generation for PPTP VPN server'. When I was talking to Manuel about this feature request he pointed out that it would be better to implement this feature as a hidden config option, since the GUI has already too many options (in some places). When this feature will be implemented, you should be able to add firewall rules to allow/block pptp traffic from specific IP addresses.