if you select any/all files clt-a on the Audio window, or ctl-select individual files if you want to do a few first, for any reason, then in any window that shows the ratings, the tagging window, or in the view scheme window showing the rating column, move your mouse over any track that has a rating, you'll see the stars darken and if you move to the right, it'll show you and X. when you click on that X, it will remove ratings from every track you have selected.
done.
