All frustrating stuff.
Maybe clubs could consider adding a surcharge to the entry fee, say £3, which is handed back at registration with the runner's number. (Pays for the first post race pint). I know it's more admin and a pain, but fell races are so cheap (God bless you all) that many people seem willing to enter on the understanding they might not bother turning up. Those who can't turn up for bona fide reasons would lose out, but for a keen runner that's probably only once or twice a year and they'd have a better chance of securing a place if the scheme deterred those who are simply booking an option on a place.
I wait to be shot down in flames.