Users have created a ton of batch input sessions and processed them with SM35. The users did not care about deleting unwanted sessions and there are a lot of old sessions, with almost any status (in processing/in background/processed/errors). All of them are junk (in your terms), and have to be removed from the system.
You can delete unwanted sessions with abap program RSBDCTL6 at once and then setup a background job that deletes the sessions for a specific session, user who created the session, date range, older than x days etc. that runs on a regular basis.
As an alternative, you can run program RSBDC_REORG. However, our tests showed that this program may not be suitable for sessions that have a status other than processed.
Note: This has been validated on a ECC 5.0 system. Please consult SAP Marketplace if your system doesn’t have these programs and/or if these are suitable for your release.