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But how if moderators give different random questions for each new member and the list of registration questions will be changed every month or week. I think it can help although just a little bit.
You just can't imagine but those who earns their living here by spamming and farming, they would solve this problem, with a little bit of coding, most of the systems like this can be easily bypassed.
There's no use changing the questions, because as soon as an answer has been found, the bots will easily answer it from the very next moment, so we need something really different.
I know that IP address can also be a unique identifier, but VPNs are hiding the people's real IP address and also a few people, using the same VPN service provider can share the same IP address, so that's also can be tricky to rely on the IP or not...