DPM protects tapes that belong to a protection group from overwriting or 'mark as free' for the configured protection period.
This is a useful option and protects you against accidently overwriting tapes that should not be overwritten. But in a scenario your testing long-term protection or you just want to reuse tapes earlier, this can be a ignoring.
There is a work around for this by using a Powershell Script:
This is posted on the DPM blog site