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?

you can probably do it, you will have to go in and change code, there is no 'option' or setting to do it.
The code that shows the tags is in bfa_postinfo.php - lines 245 to 292 in version 3.4.4