There are four major plugins as of writting of this that can be used to reduce significantly the amount of registration spam in Elgg 1.8.x <= (1.8.3)
Probably there are other plugins to protect against spam in elgg, however I personally tried just this ones to work with elgg 1.8.3..
1. Elgg Anti bot spam registartions with Text Captcha
As you can see in the picture this plugin requires, skills in maths For serious websites it also looks a bit ridiculous, besides that is actually an easy one to handle by spam bots, probably plenty of the nowdays spam bots crawling the net could trespass it.
2. Protecting elgg registration formw tih Image Capcha
3. Elgg anti registration spam with Google Captcha
4. Just a Captcha for Elgg 1.8
- Check and Download Elgg Just a Captcha for Elgg here
One note to make here is the 4 Captchas did not work together if enabled from elgg administration panel. You will have to use one at a piece.
I haven't tested to I don't know which one is the most efficient. Anyhow I really think Image Captcha is looking best from all of them and more intuitive to the user.
I'm quite happy Image Captcha is available and works fine in 1.8.3 in prior version 1.6.x generation, I couldn't find any decent plugin to filter login spam and my experimental social network based on elgg, got quickly filled with Spam. Now will wait and see if the Image Captcha will stop the drones.