I am going to close this issue since I think it applies to the 
bugs.python.org tracker and they are shifting to github issues
since their source is on github.

However implementing a silent submit button (trivial change) is
documented at:

  https://wiki.roundup-tracker.org/SubmitSilentChange

Also sending email on a nosy list change isn't the default behavior
IIRC. Usually you need a change note. My guess is you implemented
some form of:

   https://wiki.roundup-tracker.org/NosyMessagesAllTheTime

which also describes how to not send nosy messages for particular
types of changes (note this page hasn't changed since 2009).

For your specific case something like this at the top of the 
nosyreactor:

  silent_fields = ['nosy']
  try:
      for v in old_values.keys():
          # loop over all changed properties
          # if any one of them is not in the silent list,
          # raise an exception to trigger sending a nosy message.
          # otherwise return without sending a message
          if v not in silent_fields:
             raise ValueError('found non-silent changed field')
          if v == 'nosy':
             # do stuff here to see what changed and if the user
             # removed/added is the same as the user returned by
             # db.getuid()
      return
  except ValueError:
      pass # we found a property that should be reported

  # do stuff to send nosy message

might do the trick.