We don't provide a feature for that. Furher more, captcha only appears once, upon registration. If you are logged in, you don't have to pass a captcha to post. The captcha is VERY light, less than 5Kb. If your users can't afford that once, I bet they cant access your forum at all. The captcha is not javascript or flash based, it's pure html.
Maybe at premium support you can ask for that since we would have to change the code to do that. Anyway, I strongly advise against it and I don't see any reason to do it.
Nabble staff. We never ask for passwords.