I asked Ash about this a few days ago and he told me spam blacklisting was built in to Pligg by default, just not enabled. I hadn't noticed this until he told me. I haven't tried it yet, but it should block spam from a global blacklist. I'm sure he'll give us some more details when he sees this post.
To enable Spam protection Open config.php Find: define('CHECK_SPAM', false);
Replace with: define('CHECK_SPAM', true);
I started a
wiki page for documentation so others can contribute.