Update 14/04/2011 - SPAM Moderation Plugin has been updated to support keyword based filtering, and Mollom SPAM filtering service is also integrated. Check out the github repo.
Today I started by journey with Elgg. I wanted to get used to the functioning of the system, so I was reading the Docs (http://docs.elgg.org/). When Mr. Akshat, asked me for a plugin that could do SPAM detection for content posted by students. So, I thought its time I dive in for some action with Elgg, and here is the outcome of 3 hours of Coding - A new SPAM Detection plugin for Elgg using Akismet web services.
To add a feather to the work, I feel so proud to say, this is a proposed project idea in Elgg for GSoC 2011. I do not know, if anyone is working on it. If so, please consider this to be your ground work.
I'll post it on my github and will also submit to Elgg community for feedback. Also note that this plugin is in pre-alpha stage. Only the blog posts are checked for spam. I'll add more content check in the upcoming days.
GitHub Repo - https://github.com/ashwanthkumar/spam_moderation