Multiple Categories Removing Just One

First a small history, I figured out (much later) that you could link to all posts by one author.

As such i now have this problem, before i knew you could link to all posts by one author i was using a category system. I would simply place each post by an author into a category with that authors name.

I now want to move over to simply linking the “by soandso” to the list of all posts by that author.

The main question is, If i have a post that has multiple categories such as
Bob news environment

Is there a way/method to go in and delete the “bob” category and simply leave the post in
news environment

I ran a test and found that it puts the post into
uncategorized news environment

I could rename the default cat to something better but i just want to be able to delete one category and leave the rest in without adding the uncategorized category in.

Some of my authors have 500+ posts and it would be such a pain to go in and unclick them all

what i really want to do is just delete the author categories and default to the other categories already on the posts.
any suggestions?