Around 2 weeks ago, I noticed a flood of Viagra mails getting through my SpamAssassin filters. Turns out that the spam was using Habeas haikus in the headers - and thus SA was giving it a -8.0 score.

Ramping the habeas rule in my SA config up to 0.0 stopped them.
You need to put this rule in your systemwide SA config:

score HABEAS_SWE 0.0

Goodbye "Habeas" emails.