How do I delete just the Time Machine files on my Drobo external filesystem?

I have a folder on my Drobo called "HFS+ Private Directory Data" and it contains old Time Machine data. I cannot delete it and do not want to reformat my drobo. I have tried 'option' trashing and various Terminal hacks but to no avail. I am using Lion now. It's about 36 GB and I would love this space back. I'm running Lion on a 2006 MacPro and it's a four bay Drobo with 3 TB of 4 TB used. I use the Drobo for image files and a separate drive for Time Machine. Any ideas?