I'm running elgg 2.x

I installed this "emaildomains" plugin: https://elgg.org/plugins/1422171 

And this "elgg_recaptcha" plugin: https://elgg.org/plugins/2368593

And this https://cleantalk.org (free for a week then £6 for a year so it's very affordable) and have had really good results.

Combined they have dramatically reduced the appearance of spam and the questionable registrations.

I have not tried this plugin but woud be interested to hear if from people that have: https://elgg.org/plugins/774755