RFC 7505 describes the accepted method to add a DNS record that will tell requestors that the domain is not in use for any email. The method says to create an MX apex record with the site name '.'. Currently the DNS manager will allow you to enter the information but will not save the record with a period for the name. For example

".example.com IN MX 3600 0 0"

Please see Request ID: 00866206.