VPN + hotspot

Hi

I have a mikrotik router (X86 based) working with hotspot authentication over an existing wired campus network.

I need to add access points in different building and link them to mikrotik box.
First, there is no possibility to do VLANS

I am using a DD-WRT access points (PPTP, EoIP capable…).

The idea is to enable PPTP server on mikrotik box and to connect AP as PPTP clients. this configuration works and i can access to the internet from the different sites. but I don’t have any security because access points are the PPTP clients and users connected on these AP can have internet directely.

So there is a way to force PPTP clients to be redirected to the login page ?
or there are any other idea to do this ? (a configuration similar to chillispot with a centralized server). Plz don’t propose to buy 100 mikrotik routers !!
thank you for your suggestions !

Chady

so ?? nothing ?