I had the same problem and was equally frustrated by the Comcast site. Here's the issue I had - maybe it's the same as yours...
- The comcast ID I've used for the past 8 years apparently is not the primary user ID - login and find your way to the self-test page (xfinitytv/comcast/rh) to check out what it is according to Comcast's system
- Avoid the telephone helpdesk at all costs - go via chat with an analyst - faster and less frustrating
- The analyst will guide you through how to access your primary ID (or make whatever you login with your primary one) - takes about 5 mins (you'll need your PIN - or they can provide/reset one)
- Once you do this, it will take you to the same device management screen (https://device.comcast.net/devices), but this time, a link shows up to remove the devices!!
Adding the one line 'NOTE: You need to be logged in using the Primary ID before you can change the users on the app' would save a lot of time, effort and hassle. Just sayin'
Hope this helps others.