Akismet è gratuito solo per uso personale e per un unico sito, mi pare comunque non vincolante: “Install it as a normal admin, activate and add your key.
Deactivate and go to your server file manager /ftp and move it to mu-plugins”
It’ll activate on each new blog under the key in the database, or at least, it did last year, they might have tightened the code by now.
If you’re going to do that, make sure it’s on a site where only you will be creating blogs, and add the extra plugin to put the akismet “powered by” link in each blogs footer.
Una alternativa gratuita, almeno ora in beta, è Typepad
altri consigliano strategie differenti:
http://wpmututorials.com/how-to/stopping-spam-comments-sitewide/