I wouldn't worry about erasing the removeable disks, you don't erase a tape when you reuse it, right?
Use media set OPPs and APPs so that the data is protected as long as you need it, and then let BackupExec reuse the space on it's own
If you use Append jobs, remember that the OPP is reset each time a new job opens the BKF media family, and that all BKFs share the same OPP. This means that unless you run periodic overwrites (weekly for example) eventually your disk will fill with bkf files none of which will expire for another xx days as set by the OPP