Questions aren't an effective way of doing it as they are easily answered and automated and no one wants to go researching complicated questions when they are new to a community.

The only way you would prevent the current situation would be limiting logins. But then the bots would out way the amount of real people that are trying to sign up and would hinder the actual users.

I don't know what can be done other than removing signatures and banning bounties. Thats the only real solution which would prevent 90% of our problems. Yes there will still be spam that not to the degree which it is currently. Lets be honest signature campaigns have ruined the community. I don't really blame users wanting to earn money but at least put some effort into your "job".