You can manipulate the field using an edited mpl file, but I have not been able to clear it this way, only change its contents.
I tried a simple space, but it didn't take.
I tried 0, and it made the field literally, 0
I tried <empty>, and it did nothing.
I have 12 files in my cache with "Conversion", the other 8000-odd files are all "HH Conversion".
I removed the stack tag jmd file from the library directory. The stacks appeared to remain intact, all the stack tag info was gone. I did not then follow this on with a hand held sync. I guess this would recreate the library file, but I'm not sure how MC would handle the sync.
-marko