I was having a similar issue and I if I understand correctly, I think this is a solution for others..
in itunes with iphone plugged in, go to summary tab. click "manually manage music.." and apply. Then go to "on this phone tab" and you can actually physically delete the voice memo playlist or individual voice memos. This appears to have finally worked for me..or at least the voice memos playlist doesn't persist under the "on this phone tab" anymore....not sure it made a huge difference in my iphone available space.