The key concept is same as with CAPTCHA: user is presented with test question or instructions and must give correct answer to use resource. Main difference is that computer does not try to automatically generate "unique" test questions on each query; only verification of answer is automatic. Instead, unique test question and answer[s] are set by moderator or owner when SAPTCHA is installed, and are changed every time spamming happens.
Example questions: User is given instruction like "write [no i'm not a computer!] in this text field" or "write 'i'm human' in reverse" or "write[or copy-paste] web address of this page there" (please don't use too similar things. No default questions and answers. Think up something yourself. Don't try to be clever. It should be not more complex to understand and do than rest of registration, and thus shouldn't decrease website's accessibility(!). It's better if answer is more than 1 character long.)
