Metal Captcha
// edit: sense of humour failure. I care too much about blocking spam :)Sorry, but this is harder for humans than bots.The logos are quite distinct and not randomly deformed, so it's easy for even a simple image recognition algorithms to tell them apart.And the set is very small (for this problem anything smaller than millions is very small), so it's feasible to manually tag enough images for bots to brute force it.