Problem with ppp keeping link up

[bmward@inet.net.nz]

I have a Debian gateway system, doing pppd demand dialling. ISP gives 
a new IP address with each connect. No problems with that - pppd works 
as advertised.
My problem is that once up, the link often does not shut down even when 
there has been no traffic from me for longer than the set time-out 
period. 
The reason appears to be traffic coming in from outside. It looks 
(Ethereal) as though it is traffic for a previous user of the IP address
I now have.
My (simple) iptables rules ensure that my machine does not respond, 
but the traffic is enough to trigger pppd into keeping the connection 
active.
I note that back in Jan 2003, someone had the same problem but got no 
response on the debian-user mailing list. 

How can I ensure that traffic initiated from outside does not keep 
the dial-up connection open?

Bruce
Ward