Hi Juergen L,
It is possible to delete the archive files from the open text or any third party ILM certified store directly via standard ILM functionality.
You even don't need to go to open text for deleting these files and ILM will take care of that but the main pre-requisite here is the retention period. Until the Expiration Date of the file has not expired you can't delete that but once it is expired , the data will be visible in the SAP standard transaction ILM_DESTRUCTION.
From there you can create a worklist and delete it. This will destroy all the related data from database tables as well as from any external store.