@Gujyot: AFAIK, sending sms is not free. Admins will have to pay for every single spam sms sent. Not to mention you're leaving aside users who don't have a phone
@The One: Although I do think the idea is better than what the core currently has it won't be difficult circumvent. From the spammer point of view they need to write a simple question OR take it from a question database when registering OR just a simple comment saying "Thank you!", as I guess when you said "question" you meant "post". The rest of the process is exactly the same for them.
From your point of view, you will see users registering with a question (related or not to your main forum, depending on the spammers post DB), answer or comment. Now, if the question is unrelated, does that mean the user is a bot? If the post is a "Thank you" does that mean it is a bot? You won't know and you will end up approving the user.
Of course, this should work for the first wave of spammers after this new system is implemented but it won't work for the second wave (a matter of hours or maybe days later?).
What other alternative can you think about?
PS: As a side note, can any of you guys publish a dump of the users table? Not the whole table, just the username, email, create and login IP address and personal fields of the spammers. Ideally, if you can include a separate dump of the posts those users have created, that would be great