Host/Network Blacklist not working (trying to block spam from .icu domains)

 We get lots of spam from .icu domains. Eg: pepper@coupair.icu and value@wheatsolo.icu

In Antispam/Sender Blacklist I have: *@*icu as a 'Blacklisted Address Pattern' and that does not work. Reading the online help I now know that "A wildcard does not work in the domain or TLD part of an address." Grrr.

So in Relaying tab, Host/Network Blacklist section I put a DNS Host with hostname *.icu but this doesn't work either. I tried adding a DNS Group here with same hostname and that doesn't work.

Any ideas how I can stop these as early as possible?

Thanks, James.