You can specify the networks that can be accessed from pptp users when creating the IP rule.
on the destination of the IP rule specify the interfaces you need to be accessed (if lan is specified, then only lan interface will be accessed, if you want more than the lan interface to be accessed then create an interface group whith the interface you need to be accessed). if you want more than the lannet to be accessed then create a object group with the lannet and the others networks to be accessed.