Pollywog writes:

However, if you dspam remains too difficult for you, then three things:

1) Spamcop-URI (this functionality is included in SA 3.0) will catch bad URLs in the mail. It takes some hours for a new URL to be added, so spammers get new URLs every day or two, but it catches a lot of spam.

2) Use an RBL (at MTA or Spamassassin level) that blocks dynamic IPs. Almost nobody on dynamic IP runs a legitimate mail server (Softflare has a few customers that do) and most spam comes from zombie home computers that are on dial-up or adsl/cable with long dhcp leases and are classed by RBLs as dynamic (technically there is no reason why these shouldn't be static but ISPs can charge more for a fixed IP if the standard product uses dhcp).

3) Spamassassin rule (in local.cfg or any other .cfg file):

describe YOUR_RULE_NAME No Subject header YOUR_RULE_NAME Subject !~ /\S/ score YOUR_RULE_NAME 6.0

Adjust the score according to whether you want to definitely block all of these or want it to come close to your trigger level so that any other rules triggering will take it over.