All I see under "Email is this:
I think he means you can do this at the end-user cpanel level, maybe you are looking for a server-wide fix, then that would be a ACL rule in exim, something along the lines of a deny statement.
deny message = This TLD is not allowed to email to us
(can someone verify its really that simple and wont kill other ACL's?)
Just a note if this helps, alot of spammers are not putting their domains in the spam they send so while the .ru domain is a killer, you will find they often fake the domain or use a known-good one to send. Doing a "tld" level block will help some but you still have to do the initial handshake, get the email address and then do the reject and drop the connection.
Implementing a three-way method, that is blocking based on IP address ranges, RBL checking and a TLD reject will be the best bet.