In the past we've recommended getting the IP# subnets and putting them into the bad_sender_hosts_ip file.
There have been some recent posts on a suggestion to using a different exim search pattern which allows wildcard exim searches. Unfortunately I don't have time to look for it right now, but perhaps you can find it.
If you use that instead you should be able to block something like *.asia. If you find it let me know, I'll consider making the change part of DirctAdmin's standard install.
Jeff