Can you mark a contact to not receive emails without removing their email address and making sure it does not receive any emails in a campaign of any sort
You can create a list of email that will not receive an email and make a comparison overtime. But the easiest way is to not include them
You can go to the contact record, Marketing tab, Subscriptions subtab. Make sure you select Soft opt out. This will prevent them from receiving any campaigns. This will not stop one of your users from emailing them directly.