Well, if you exclude a category, then it works both ways: the blogposts marked in that certain category will be excluded from showing up in your related posts as well as that those blogposts won’t have any other related posts, no matter from which category they are in.
In a sentence: no related posts in blogposts from excluded category + no related posts from excluded category.
The best thing to do is to mark those blogposts you don’t want to show up in a new, special category and then exclude that one in your plugin settings.
Let me know if this is what you had in mind or if you have any more questions! ??
Take care!
Silvo