Being charged for two boxes I don't have

When I changed my service online a couple of months ago (to replace HBO with Netflix), I had to go through the "new service" procedure for some strange reason, and, as a result, I had two boxes that I did not need added to my account. I returned the boxes to the nearest Comcast office the day I received them, but I noticed that I had been charged an additional $40 - $20 for "service to additional TV with AnyRoom DVR," and $20 for "service to additional TV with TV Box Qty 2 @ $9.95 each".

I called customer support and was promised that the boxes would be removed from my service, but I just got my bill for September and the boxes - and the extra $40 that goes with them - are still there.

What hoops do I have to jump through to have these charges removed from future bills, and, preferrably, have the current charge credited as well?