MommyZabs has told me about the If You Liked That plugin which works for her instead of Related Posts.

You can edit your robots.txt file (found in your root directory) to say:
User-agent: *
Disallow: /category

That prevents Google from indexing your category pages (although for some reason Googlebot disobeyed mine until I changed the user-agent to Googlebot).

Is there another plugin for related posts?

My biggest problem is that Google has indexed my site by category rather than by post. So when someone searches for something specific, they have to scan through a page or more of different posts to find it. Often I have exactly what they are looking for, but I don’t know if they ever find it.