I'm pretty sure you can't.
Of course, it would be technically possible for someone with database access to go in and permanently delete a post. But no such feature is exposed in the UI, up to and including users with administrator access.
Deleted posts are also only visible to a subset of users (bugs notwithstanding), so this isn't a big problem in practice.
I have seen some cases where the owner of a post has edited it to delete the actual content, when duplicates were posted, and simply indicate something like "this was a duplicate of <some other post by link>". For situations where the content is truly a duplicate, and especially where the post is long, this is probably the best (or least bad) option; but do note that it still does nothing to change the licensing status of the originally posted content.
Another at least potential issue is that such a permanent deletion feature would go contrary to the fact that the CC BY-SA 4.0 license is irrevocable. (My emphasis below.) At the very least, such deletion would make it more difficult to show that some content was provided under some specific license, as also to some extent discussed elsewhere.
Section 2 - Scope.
a. License grant.
1. Subject to the terms and conditions of this Public License, the Licensor hereby grants You a worldwide, royalty-free, non-sublicensable, non-exclusive, irrevocable license to exercise the Licensed Rights in the Licensed Material to:
A. reproduce and Share the Licensed Material, in whole or in part; and
B. produce, reproduce, and Share Adapted Material.
(When you post to Codidact under that license, "You" includes Codidact as the initial licensee.)