There are plenty of ways, getting an email bounced back is a classic way to simply remove a listing regardless of content.
Free directories get mountains of crap, I simply look at the title, if they have not followed the guidelines, it is rejected regardless. If the email comes back as undeliverable it is automatically removed from the index.
There are plenty of ways to sort the wheat from the chaff, but I doubt you will get directory owners posting a 'how to beat my spam systems' type post
