Tentative resolution:  For all resources, the following changes will be made:
When the STOP method is issued,
  a)if there is no request active, the server will return a 200 OK without an
Active-Request-Id-List since the resource is in the intended target state.
  b) otherwise, the server will return 402 Method not valid in this state.

The history/discussion for this issue can be found in the thread beginning with
http://www1.ietf.org/mail-archive/web/speechsc/current/msg01770.html.