yes it is possible easily .
first : i dont know your router device model !
but usually you can run MikroTik User-Manager for create your user (Use it as Radius Server) then you can easily limit RX/TX ban-rate and Limit Download Traffic for each user per month or day.(dont mater you use PPTP Service or any other service for Access server such as Hotspot)
this is one solution of thousand solution ! for example your other solution is using Microsoft ISA Server !! very simple and very powerful for bandwidth splinting ! or you can run PPTP Server on your Mikrotik Router and use ISA Server as Radius server
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