Outside this forum, this is a well known problem with BT on the DNS-323 - since its introduction - see other fora. Try this
http://forum.dsmg600.info/search.php
In short, as you have found, it is not possible to delete or to move anything from the BT directory by normal means. You can copy these files to another location. It seems you have to 'telnet in' with intergalactic privileges (777 is mentioned) to move or to delete files. As I found out, my Linux knowledge does not extend that far.
I eventually reformatted the drive to remove these directories. After doing that a couple of times, I changed back to leaving my main PC running. Not electricity power efficient but less problematic.
It is what has prompted a number of us to use ShadowAndy's mldonkey with sancho.
Biscotte