Can't Delete Files From Zotero
My Zotero storage was almost full, so I deleted an old Collection that had a bunch of papers I didn't need anymore and emptied my trash collection. However, my Zotero storage hasn't decreased (it's actually at 100% now...), and all the files I deleted are still in My Library. It's been about 30 minutes since I emptied my trash collection,
Upgrade Storage
https://www.zotero.org/support/collections_and_tags#collections
If you wish to delete all the items within a Collection you would select all the items there and then right-click Move Items to Trash. Then empty your trash (or wait for the #days under Edit\Settings\General\Miscellaneous).
If you still wanted to delete just those items, you could have restored the Collection from the trash, and then selected items/moved to trash as above. But as you have now emptied your trash, the Collection is gone so you can no longer do that (you could however restore from a backup).