I want to block torrent and other file sharing apps from Airport Express. How could I achieve this?
I am not asking the port numbers.
Advanced > Port Mapping doesn't allow me to specify port ranges.
Incoming connections are naturally blocked by the NAT. AirPort Express does not provide a way for you to block outgoing connections.
Some Mac torrent clients such as Transmission use NAT-PMP to dynamically create port mappings so they can work. If you want to manually manage all incoming port mappings, disable NAT-PMP on the Express. Beware that you may have unwittingly been relying on NAT-PMP to work for other Mac and Windows software, so after you disable it there's a small possibility you may have to manually configure some port mappings for some of the protocols you've been using.
But as you probably know, even without NAT-PMP, many protocol implementations can get around the inbound connection blocking that is a natural side-effect of NAT.