I need to create a spambot protection measure like they have on phpbb, which works better than anything, cos all these IP blocks is a Micky Mouse job, they don't work, new proxies come out every day (if not every hour). 

This should be a question at registration to which users MUST give a correct answer. For example, what is the capital of France? Or (better, which is actually what I want) a math task. For example: 48+5=? For real ppl it is easy to give a correct answer, but for a bot it is a huge problem. And once the bots figure the correct answer, you -pah! - change it from admin panel to another task, lets say 32+9=? This is like a secret password for real people to enter the site, and you can change it every week, every day, every few hours, as often as needed. 

Works like a charm, on my very busy phpbb board I have NO spambots, only an occasional human spammer. Anybody can do it? Would be nice if you can make it as a plugin for everybody, so others can benefit from it, too.