You need to finalize the media before you can purge the files. You can set a flag for this to happen automatically as a property of the Media Group. The option is called Auto-finalize DVD+/-R media after marking full.

From the admin guide:
"Finalizing media makes the media more stable, better protecting the
data on the media. It also allows the media to be taken out of the
DiskXtender system and read on a computer with the Windows XP or
Windows 2003 operating system.
Finalizing media allows the files written to that media to be purged
from the extended drive. Those files are not purged until the media is
finalized, due to the potential instability of unfinalized media."

Let me know if this helps.