There have been number of requests to allow cleaning old builds, to list a few:
Those threads have been closed automatically due to lack of updates (or discussion cleaning following the CircleCI 2.0 release). However, this feature has not been implemented or is not available, so I'd like to re-store the request here.
First, this is a desired feature especially for teams using topic branches heavily (eg. w/ GitFlow, etc.).
Second, despite mentioned challenges to implement such feature, there seem to be agreement from CircleCI team members about validity of the request.
Finally, the backlog of comments to the linked threads suggests there is a demand for it from CircleCI users.