Hi,
Is it possible to delete an individual post once it has been starred, tagged ?
I happen to have many duplicates from a feed that keep changing his url. 
A.
One way could be to untag and unstarred the post, but does it means it erase also the post from the database ?
TT-RSS already handles duplicates but if the site owner is re-using the GUID or link re-posting the same article under a different URL this won’t work.
If you star something it will never been deleted. Tagging has no effect on deletion, to my knowledge. If you want something to eventually be deleted, unstar it and the normal purge process will cause it to be removed in time (60 days is the default).
You can manually delete articles by first archiving them, then in the Archived feed selecting and deleting them. However if the article still exists in a subscribed feed it will simply re-appear.
If you’re using PostgreSQL there’s a heuristic plugin that tries to find duplicates across feeds but I don’t think that will work in your case. Your best bet is to contact the site owner; I’ve had quite a bit of success simply letting the owner know there’s a problem.
e: Depending on the content, etc. you could setup filters to delete or mark as read these articles.
Thanks for the quick answer.