In order to scratch tapes manually, I updated the EXPDT with today's date and cleared the OUTCODE. However, the next Vault Management run vaulted the tape again and thus it was not scratched by TMSCLEAN.
How to prevent tapes from being re-vaulted?
Run TMSUPDTE using the following control statements to update the OUTCODE with ' VMS':
//TMSUPDTE.SYSIN DD *
REP OUTCODE=' VMS'
Note the blank before VMS in the above control statements.
OUTCODE=' VMS' indicates, that the tape has been returned from the vault and will not be processed by Vault Managment System (VMS) again.