One trick is having an tarpit email adress on your website. It is hidden using CSS so no real visitor sees it but it is visible in source. If your mail server recieves mail for that adress you can just block that IP for 24h.

This sounds like bad advice and would result in blocking google and other major ESPs.

I occasionally get spam from people who took the time to create gmail accounts. Based on this advice, the honey pot email address would get spam from a Gmail account and your script would block Gmail servers.

Similar in spirit to this: https://www.projecthoneypot.org/