I have a ReadyNAS NV+ and I had the same problem. I worked with Netgear tech support and they could duplicate the problem from the Mac side. They produced a newer version of the firmware so make sure that you have the lastest firmware update for the device.
Additionally, I found out by accident that accessing the ReadyNAS NV+ using WinXP running on VMWare Fusion on my MacPro, I was able to delete the files that I could not delete from the Mac OS. So if you have Windows, either with Bootcamp or emulation software, you could do the maintenance.
North Wales, PA