I have a form which we wish to rate limit, we've decided to use a captcha and always present it to the user. We don't care if they are a bot, or not, we want them to have to enter a captcha regardless. It doesn't matter if they use the form once, or twenty times, each time they submit they will have to enter a captcha.
To this end I've modified captcha to add '#captcha_always', if it's set to true it will always request a captcha from the user. I've configured this in the specific form I wish to use.