[exim] how do i filter against utf-8 headers

I noticed that the majority of the spam that my mail server receives has a
subject that is UTF-8 Encoded. I want to deny ALL emails whose Subject
line contains a UTF-8 encoded stuff.

I tried this, but it does not work:

   deny    hosts = +rbl_hosts
           message = UTF-8 not allowed
           log_message = UTF-8 not allowed in subject: $h_subject
           condition = ${if match\
           {${lc:$h_subject:}}\
           {\Nutf\-8\N}\
           {yes}{no}}

That does not successfully match a UTF-8 encoded subject line.

What's interesting, is if I look at a spam email within my mail reader
(Pine), it looks like this: Subject: [utf-8] Hi again

But the actual text within the /var/spool/mail file actually looks like
this: Subject: =?utf-8?B?SGkgYWdhaW4=?=

So there's some kind of encoding magic happening here that may be making
it difficult for me to filter against UTF-8 messages. Any suggestions?