Generally using a wildcard character, example 208.93.7.* or 208.93.*.*
However, one of the devs might be able to correct me if I am wrong with Phex.
ie: this is how it's done with LimeWire & some other p2p sharing clients, so I presume it's the same for Phex.

Keep in mind, blocking a wide range may affect innocent users who are not using such clients or versions as you suggest. And ip address ranges are not strictly divided into country or area zones. PM me if you wish to find out country ip listings.
Also keep in mind, lots of people nowadays have dynamic ip addresses which may switch from totally different ip addresses you may initially ban. I know my ip address switches frequently each week. I've noticed same thing about some problematic gnutella users (yes recognised them as the same persons.)

Very late Edit: seems I might have been incorrect about the above wildcard for even LW.