Are your jobs APPEND or OVERWRITE? if they are all APPEND, none if the BKF files will ever expire, since they all share a common OPP date, which is reset when the latest file is closed
For Disk based backups, I recommend all Overwrite, for just this reason
Also from Options/Media Management, make sure that the option to "Reuse Overwriteable media in the target media set before scratch media" is selected. If the other option, "Use scratch mediua before ..." then backup Exec will continue to create new BKF files until the disk is full or the Reserver is met