Tony, a capture is a must nowadays. It's as simple as that. But unfortunately even simple captures are hardly able to keep bots away nowadays.
I hope you don't mind me mentioning this ... checking everybody is not something you (personally) should be spending your time on, IMHO. Either outsource this, via Elance for example, to some guy in India who will be happy to earn 10 bucks extra per week.
Or find somebody who is willing to do it for free in exchange for a ChessPub subscription. That's the only currency you can print in unlimited numbers.
An idea:
Instead of asking for a reason, ask for something a bot cannot know:
Ask a long question and have the answer be a specific chess term, for example the German word "Zugzwang", or "Zeitnot" / "time trouble". That's impossible to guess for a bot without human help. Or ask "Who is the most famous American world champion?"
In any case the answer is short and easy to process.
Maybe a reversal of process would also be better. I'm a mod in another forum. We let everybody in who proves his intelligence by getting past the capture. The software generates a page with the latest registrations. Here everybody is assessed by hand. People with suspicuous usernames or obviously faked profiles are killed immediately, without warning or communication. The (custom coded) sub-page allows to instaban all marked accounts with one mouseclick.