typically considered the number tool in preventing user signup spam. Google it and implement it if you want to add a layer of difficulty in user sign up.
Re custom stuff, I hate to break it to you but spamming has largely been an automated job for ~10 years. The code is constantly evolving and getting more sophisticated, possibly someone has just added a plugin for the type of forums that you're running here, or given how old and generic it is, someone has just pointed it at this URL and the package is doing the rest. It's unlikely to be personal.
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