Asking questions, especially questions specific to the forum domain, seems to work better than visual puzzles (CAPTCHAs) at stopping both spambots and human spam farm workers from registering. Some other visual puzzles (e.g., show an ornate analog clock face and ask what time it is) still seem to work fairly well, but regular CAPTCHAs ("what word is this?") seem to have been pretty well cracked. You just have to be careful when designing questions that they will be answerable by your intended audience (usually in one word, with one correct answer), yet be unfamiliar to Third World spam farm workers.
P.S. Microsoft doesn't have an "owner". It is a corporation and there are millions of stockholders. Do you mean "principal co-founders"?