On Marketing > Email > Contacts click a contact's #ID or Edit to change its Name, Email Contact or Receive Email? toggle, then Save. On the same page a red Delete button (confirm Are you sure you want to delete this contact? Your data will be lost) removes the contact permanently, including from every list it belonged to. To delete several at once, tick the checkboxes in the Contacts table and use the bulk delete action that appears. Registered customers cannot be edited here – manage them under Customers.