Why would you want to? Media protection/managment is the primary reason you spent the money on BackupExec rather than just using a spreadsheet, right?
What you can do is set OverwriteProtectionPeriod on your media sets, so that when tapes are due to be reused, the OPP has expired. For tapes written the same day of the week every week, use 6 days
Also, since you have to inventory everytime you swap tapes, if the tape header does not show in blue, drag the volume to the scratch media set, if it indeed is OK to overwrite that tape.