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.
info@elgg.org
Security issues should be reported to security@elgg.org!
©2014 the Elgg Foundation
Elgg is a registered trademark of Thematic Networks.
Cover image by Raül Utrera is used under Creative Commons license.
Icons by Flaticon and FontAwesome.