Unmonitor file after successful download

With disabled option “automatically import completed downloads from download client” I would like to have possibility to automatically unmonitor files which have been downloaded.
If not instantly then maybe after 24 hours or whatever, something like expiring after it goes to the download history.

Edit:
By looking in API and Custom Post Processing Scripts I see there is a possibility to almost do that, but I don’t see the event for completed download.

That is exactly how you would do it.

Settings > Connect > Custom Script > On Import

That will trigger once it is imported and you can then use the API to unmonitor it