From Talent Pipeline, you can now reject candidates directly from each candidate card (which immediately are reflected in Greenhouse as well), and view all rejected candidates in each stage.
To reject candidates, click the arrow in the upper right of any candidate card --> Reject Application:
To view rejected candidates, in the Active Jobs view, hover over the name of the job in each job section, and check the box to "Show rejected candidates."
In the Consolidated view, check the box in the upper right of the screen to show all rejected candidates.