>After that we make best efforts but if people can’t respond to vendors they pay money to, we’re really at a loss

This is going to sound counterintuitive, like the Birthday Problem or Bayes Rule but at least for me it's true: most of the spam that gets into my inbox is from vendors I have a relationship with. Email isn't always ideal. Did you consider doing any other methods, like turning off writes a day or so before reads were disabled? That would trigger a much more immediate "oh shit" response than an email (unless the subject line is super clear and informative, and the email doesn't go to my span folder)