If your pptp users are on another different ip range from the lan users then just change your masquerade rule to specify the src-address to be that of the lan ip range. Then the pptp users wont have internet access at all.
set your PPTP pool to a contiguous range say near the high end and then create a firewall address list called pptp which will have this address range specified (i.e. 192.168.0.200-192.168.0.254).
Then in the masquerade rule, use the src-address-list but put a ! before it indicating any IP besides the pptp addresses.
I would like to buy a book showing tutorials and usage of WINBOX.
What is I disable NAT masquerading will my internal network be vulnerable or accessible from outside?