target ppp interface in queue is lost, when client disconnec

in order to limit traffic for specific dial-in ppp clients, I defined a queue with target a specific pptp interface. All works well, but when this interfaces vanishes (when client disconnects), the queue starts having all interfaces as target, and limits the whole router!

what can I do in order for the queue to target only the initially defined pptp interface (whenever this client connects and disconnects), and not having this problematic behaviour?
many thanks,
Dimitrios

either I am doing something wrong, or nobody has had this kind of problem before???