It generally will stop them. The phrase that applies is something like "you don't have to outrun the cheetah, just another gazelle". It's not about eliminating

It generally will stop them. The phrase that applies is something like "you don't have to outrun the cheetah, just another gazelle". It's not about eliminating the spammers, but about making it expensive enough - in CPU, memory, bandwidth, time, human intervention, etc. - that it's not worth them attacking you. These days a spammer could train a ConvNet to pass reCAPTCHA with > 90% accuracy and very little processing overhead if they really wanted to. The only reason it works is because the bar is "high enough" that it's cheaper to spam somewhere else that has a lower bar.