PPTP without default gateway

I have a Linux NAS file server with PPTP.

I have several clients that use Windows only (no Linux, MacOS or mobile phones).

I want all those clients to be able to connect to the NAS server only (no external resources), without installing any additional software on the client side. Also I want setup to be as smooth as possible, e.g. accepting defaults only.

Currently everything works with defaults, except because of default gateway, clients can not access anything but the file server.

Is there any option I can set up on the server to fix this.