looks like in the new v3.1 version you can do this:
Permanently Delete Quarantined files
(Requires a Cortex XDR agent 7.6 or a later release for Windows)
To help you better manage malicious files which have been quarantined and avoid any potential mistake of restoring unwanted files, you can now permanently delete quarantined files on the endpoint from the File Quarantine Details page.
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