I spoke with an XFINITY Home agent. I called 1-800-XFINITY and then said into the automated system that I was calling about XFINITY Home and that I wished to downgrade.
There is a "No XH EcoSaver" opt out code that they need to add on to your billing account in the same place that they add the monthly recurring charge for the service.
For what it's worth, I have actually grown to like ecoSaver... the monthly usage reports are awesome, and although it does make some changes that override my rules a little bit, I barely notice it now (it took it a few months though before it really learned my preferences). Hard to tell exactly how much money it's saving me, but every little bit counts!