Getting billed for an "unreturned TV box" that doesn't exist

Hi folks,

I've been having this problem for a while and can't seem to get it solved any other way, so I thought I would post on here to see if maybe anyone on here has dealt with it before or maybe someone from Comcast will respond. 

At some point last year, I started to get charged for an "unreturned TV box" - I thought this was pretty peculiar because I never changed out my equipment or anything, so I assumed it must have been some kind of glitch or mistake.  I called comcast and the tech support person on the phone apologized for the mistake and told me they would take the charges off my bill.  But then the charge didn't disappear - I got my next bill and it was there again.  I repeated this process and was told it would be taken off, but again, it reappeared.  Last month I got a notification that the charge would be increasing, and since the phone did not work I thought I would try doing this via chat.  I spoke to a representative online and they told me they would be able to make the adjustment, but I just received my bill and was surprised to find out the charge is still there!

I'm not really sure what to do at this point - I've contacted comcast multiple times and people keep telling me they'll remove the charge and somehow it never happens.  Has anyone dealt with anything like this before?  Any suggestions on what to do?  Could someone from Comcast try to help me here?