IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
On WordPress, I think you have to use a plug-in to insert the "no follow" attribute. I'm not sure that the plug-in will add the attribute retroactively to all of the existing spam comments.
One thing about it is after you have had to remove all those spam comments, you won't forget to set up comment moderation in the future! I've removed thousands of spam comments manually before, and it is a tremendous waste of time. It makes me want to put another pin in my spammer voodoo doll just thinking about it.