Exclude posts when build with --lsi?

I have some posts that are encrypted and therefore filled with random characters. Thus they don’t need to be considered as related posts of any other posts.

Currently, my workaround is to code a plugin to hijack Jekyll::RelatedPosts#build_index method to not add excluded posts to the lsi index. However, I think it is not very elegant. So I wonder if there are some “official” ways to exclude posts from building with --lsi?