I thought it was just a joke, but when I read this thread I know that it wasn't a joke.
So, this is a serious problem for the site
Pretty much every substantial site is affected by it from time to time. You'll never stop it entirely.
I always wondered if making the user solve a captcha before using the site could prevent ddos, my guess its most likely not since every site would have done it already, but why wouldnt it work? Bots that solve captchas?
Bots can solve captchas, and as far as I'm aware Satoshi created the capacha that the forum is using. (May of been changed since, I read this a very long time ago) Which is immune to most known OCR captcha solvers. But, if someone can't solve a certain captcha they just send it to a service which uses humans instead.
Captchas are useful, that's why they are used nearly on every single website. They have limitations and only prevents certain users. Although, captchas are mainly used to reducing spam and not for protection against DDOS. DDOS is sending a huge amount of requests to the server.