We do not delete individual builds on CircleCI. Even If you have accidentally dumped any secret/passwords into your build output.
After exposing sensitive information, it is too late to ensure the security of these secrets or passwords. Please practice good security hygiene and rotate any keys or passwords that may have been exposed. That way, these leaked secrets cannot be used.
Environment variables and Contexts do have secret masking enabled by default. More information on this functionality can be found on this blog post.
You can add a vote to an existing request for this feature here.