Pursue the "Gentleman" by civil means for breach of contract?
(the latter could be done by lawyers paid by AUK, to whom the fees would be negligible. A civil court claim would certainly send a message. Ideally the lawyers would take all the legwork away from the Org.).
I stress - this is not suggested as a feasible course of action right now for Mr Sheep! Just thinking of the future ...
The Board are considering the options, but I doubt legal recovery for a £5 debt would be high on my list (but we haven't had that discussion yet so you just never know). My own inclinations, as an Organiser, are to refuse refunds for DNSs, but I reserve the right to use my discretion as per 3peaker's post (but I'd be mindful of how it undermines AUK's no-refunds policy, so it would have to be good reason).
If a quick resolution wasn't possible I'd certainly return the fee paid as an interim measure if only to free up my PayPal account, but the entry fee still remains a debt, payable by some other means. No further entries would be accepted from that person EVER AGAIN for any of my Events or Perms until a satisfactory resolution had been reached.
With my Membership Secretary hat on, I'd be disappointed to learn that this behaviour was being carried out by an AUK member (which in this case I understand that it isn't) and I might be inclined to end that membership without compensation - but I need to think through the implications more thoroughly before I make that official policy.
I'd be favourable to having a list of unwelcome riders available to all other Organisers, but that could become messy - for a start we'd need to hear both sides, and I would have thought that there are legal implications in making public such a list.
What is sad is that I'm sure we're talking a trivial amount. If this person was planning to drive to the start, he's probably saved more than the entry fee in petrol/diesel not used.