Use this:

http://community.elgg.org/pg/plugins/release/824690/developer/RayJ/spam-login-filter

In conjunction with this:

http://community.elgg.org/pg/plugins/project/821368/developer/Beck24/spam-throttle-18x

Between the two, you'll be virtually spam-free.

I've never used WP, but I have used Drupal a fair bit and they have an equally hard time with spam, I think you've just been lucky until now.  Drupal has a fair number of anti-spam modules to help out on their side as well.

My thank you post hasnt come through - so here it is - thank you.