How would you expect capachas to help against UDP flood? The attack works by oversaturating the network channel. Capachas is a (terribly bad) solution to prevent the server from spending CPU and transmit bandwidth on garbage request, but these wouldn't do anything if the server have too much packets receive in the first place.
Sorry for the worst and most hated possible solution, but I thought I'd at least mention it.
Maybe too many failed capachas causes you to not connect to the IP for an hour.