We already did that David, you need to read much more than wikipedia, this is a very complicated problem, and the captcha doesn't help at all because the spammers are no longer robots, they are real, low paid people from india mostly, we already talked about all this on the other board. Read more, really.
PS: yesterday I clicked ONE button and blocked the entire country of CHINA from the software submission form, problem there is even worst. I'll probably end up blocking more, but I do have filters already that block software spammers and maybe I won't have to, the china problem had no end though so boom, there go all of them.