exclude certain tags/categories from post info items

Is it possible to exclude (hide) certain tags/catgeories from showing up in the post info items?

Say, I have a category or tag "x" that I use for personal marking certain of posts but I don't want that to show on the blog, but still be accessible via direct link, e.g. domain.com/tag/tagx

Is that possible? I guess it's a command similar to hiding categories from the category bar, but where do I add it?