When trying to mark a tape that contains active recovery points as free in Data Protection Manager you’ll receive an error like this:

The selected tapes cannot be marked as free because they belong to protection groups.
You need to stop protection of the associated protection groups before you can mark the tapes as free. The protection group that a tape belongs to is listed in the Protection Group column. (ID: 31116)

You can still force DPM to mark the tape as free by removing the recovery points. Use the following script in the DPM Powershell.

$barcode = "000012L4"     #type the barcode of the tape that shall be marked as free
$dpmserver = "DPMSERVER"  #type your dpm server's name here

$DPMLib = Get-DPMLibrary -DPMServerName $dpmserver
$tape = Get-Tape -DPMLibrary $DPMLib | Where-Object {$_.Barcode.Value -eq $barcode}

foreach ($RecoveryPoint in @(Get-RecoveryPoint -Tape $tape))
   Remove-RecoveryPoint -RecoveryPoint $RecoveryPoint -ForceDeletion -Confirm:$false

I chose to select the tapes by barcode. You can change the code easily to match your needs.

This script will delete all recovery points on the tape identified by its barcode – so please be careful…