In your hosting control panel (cPanel), under the “Email” section go to “Spam Filters”. In order to block an email address or domain from sending email to your domain you’ll need to access the “Additional Configurations” and there you will find “Edit Blacklist”.
Blacklist located within “Show Additional Configurations”
CAUTION – you will no longer see emails from any domain/address you blacklist so please make sure your entry is correct especially when using the asterisk(*) as a wildcard. For example – if you are getting spam from a domain named “IL.COM” and you create an entry for “*IL.COM” then you will never receive email from GMAIL since it also ends with “IL.COM”! In this case the proper entry would be to add an entry for “*@IL.COM”.