Very common...walkers, runners, sprinters, look & go, they have no intention. It's all a wind up to wank at home at your expense, humiliate you or cock-block your legit bookings. On the darker side ocassionally it's a gang who send a member to scope your set-up, then book in a mate to rob at a later date.
I agree with Eve cash at the door. Keep all other doors shut so 'clients' can't scope out your flat.
Also if anyone makes a specific dress request I don't allow them to see me for free in the gear before they pay. I either cover up or change once I'm paid.
Same goes for body parts. eg if someones interested in feet, they'll be covered up completely in socks and lace-up shoes like a victorian lady. I won't be wearing strappy stilettoes till I'm paid.
I'll demand £20-50 cancellation fee, but go for it, whatever amount you like. I demanded the full £250 from one guy who thought he could send me away on an outcall as I threatened to scream and wake up his neighbours.
Probably worth a warning so he doesn't waste others time?