Barred Domains

Is there a way to bar all sub-domains at a certain domain.
For example I am getting a lot of failed subscriptions from a known spam domain maybebest.com but the emails are in the format name@a.maybebest.com or name@b.maybebest.com and so on.

For now I have barred all letters of the alphabet as the sub-domain but there must be a better way?

https://wordpress.org/plugins/subscribe2/