We use EHLO blocks for something like that with a wildcard - but keep an eye on the logs for a while to make sure you don't trap something else accidentally.
We block probably about 2 dozen "nothing-but-spam" top level extensions this way.
Another thing you can do is a custom spam check that assigns those mails a ridiculously high spam score then they're blocked by the spam filters.
MailEnable survivor / convert --