How to protect html form from spammers?

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Update: The answer was accepted because I recommended KeyCAPTCHA. From my hard-earned painful expereince, KeyCAPTCHA is a scam by professional spammers. I removed my recommendations of KeyCAPTCHA

Note that most professional spambots are integrated with sweatshops (1 USD a 1000 solutions) human captcha solvers API. When a spambot cannot pass captcha itself it (spam bot), keeping hundreds of open connections, sends screenshot (or webpage code) with CAPTCHA for solving by sweatshop human. This is legal and big business. In order to be legal and integrate with bots through APIs the human solvers can not directly interact with cracked web boards (blog comments, registration pages, chats, wiki, forums, etc.).

Another problem is that anti-spam programs cannot detect context-based spamming by professionally made bot. There are many approaches. The simplest one is web scraping multi-author human dialogs from other web boards and posting them CONTEXT-SENSITIVELY (bots can detect topics) from different IP addresses of different countries at different times, so even (a weblog) owner (human) cannot detect that dialogs are posted by bots(they are really from stored in database human dialogs).

This is only the matter of interest to your website from professional spammers or time+qualified persistence of amateurs to automatically circumvent most (if not all) CAPTCHAs.

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