In our subscription center, people who Opt out completely will see a Marketo follow up landing page, where we ask them to share why they don't want to receive info anymore. This Marketo LP has nofollow, noindex set. Still it seems that somebody/something finds this page, as there are forms submitted (with the reason for abandoning) on this page, which creates records without an email address in Marketo.
Anybody smart ideas how to prevent these records from being created? Yes, I can delete them afterwards with a SC, but rather I'd prevent this from happening.
If the Thank You URL can only legitimately be accessed via the the Sub Center LP, then check document.referrer and redirect them to your home page if it doesn't match the expected previous page.
A malicious user could easily bypass this check, but then a malicious user has lots of ways to post forms they shouldn't anyway!