When you delete an item, first it goes to the Trashcan where it will stay until you empty the Trashcan. When you empty the Trashcan it stays on the file system for 30 days (default) before it is moved to contentstore.deleted. You can then delete the item from contentstore.deleted safely.
There is lots of stuff on the Alfresco document life cycle on the internet and a trashcan cleaner that removes items from trash after X (configurable) days.
To immediately remove an item without going via trash, it needs to have the sys:temporary aspect.
