[hifromwork]

I assume you never tried to add a contact form to your website.

Explanation: I did, and within a few days bots started sending me spam using that form. I just added a trivial captcha (hardcoded '2+3=' question), but if my scale was bigger that would be untenable. Think also of PM spam, autoregistering accounts to abuse free tiers, etc.

[Spivak]

I guess I just wouldn't have an open unauthed form and require a CC to use the free-tier. The contact-me form can just be a mailto: link and let the spammers go through the spam filter like everyone else. There are places where captchas is all you can really do but it's not like common use-cases don't have other options.

[hluska]

You want to put a credit card form in front of a contact form?

[graemep]

There are less annoying alternatives. Things like honeypot fields are worked for me so far. There are more dynamic variations on your maths question.