Reason for editing: changed Title and rephrased my question
At the beginning of my openhab “careeer” I had “Simple Mode” switched on. A while ago I switched it off and unlinked a lot of items I do not need anymore from things.
However, a lot of items still remain in the repository which cannot be deleted via PaperUI, via REST API nor are they stored in my .items files.
Could anybody please guide me to the correct where to delete these items?
Are all your items you want to keep in items files? If you can answer that with “yes” the easiest way is to temporarily move the items files somewhere else (or rename them), then first delete all links and then delete all items using the karaf console and afterwards rename/move the items files back.
Thanks Stefan. Unfortunately quite a few are still being required. So I prefer a selective method of deleting threm.
Do you have any idea where they are stored?
Everything that does not come from files is stored in an internal database which is stored in JSON files. The actual location varies depending on your installation method and operating system.
Look for a file named “org.eclipse.smarthome.core.items.Item.json” (OH 2.x).
Be sure OH is shut down when you change anything and I strongly suggest to make use of the backup facility that comes with the openhab-cli command beforhand so you can at least go back to the state you have now.
If you want to be sure to avoid side effects, my suggestion would be to select the item names from the JSON file but to delete the links and items via Karaf console.
The backup suggestion is stays valid even when you do not edit the file manually 
yeah - and that is the problem. All those items are not in this file. This file is empty.
Then someone with more in-depth knowledge needs to assist. Sorry
