Every new feed, creates new author – how to disable it?

For each feed, go to:

Syndication > Authors > Syndicated Authors > Unmatched authors

Under the pulldown “Authors who haven’t been syndicated before”, either choose a specific author from your user list or else add a new one to the users list via “will have their posts attributed to a new user.” (Fill in the box, and the user will be added to the WordPress users database.)

Hit save. All of the future posts from that feed will then be attributed to that one specific user.

Once you’ve done that, there should be a way in the WordPress users menu to delete the users that you don’t want, and to have their posts attributed to this one user. (But with 9000 users, switching all of them over to a single different user is going to take quite awhile.)