Hello @MarkR682. I'm sorry that you are receiving these unwanted sign-ups! One way to prevent these sign-ups would be to enable Confirmed Opt-In which would send a confirmation email to contacts who use your sign-up form to receive your emails. Only those who confirm that they want to receive emails from you will be added to your active contacts. If you have further questions about these types of contacts or about our Confirmed Opt-In feature, our Deliverability team would be happy to help you. Here is a link to their phone numbers and hours of operation.
Caitlin M. Community & Social Media Support
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