I had a similar problem with my Roamio and here's how I fixed it:
It would not give me the option to no longer have the DVR, it would only tell me there was a network problem when I selected the DVR on the my shows list. To fix it I went Online on my account I went to change device preferences. I unchecked both boxes (video sharing and video downloads) for the DVR I wanted to remove from the my shows list. I forced a "connect to TiVo service now" call in on the Roamio, twice in a row. I restarted the roamio from the help menu. When it came back on the decommissioned DVR name in the my shows list was then finally grayed out and when I selected it - it gave me the option for "I no longer have this DVR" and then it removed it from the my shows list. When I called into support they did not know about this trick because they told me it was impossible to remove it unless I take the DVR off of my account completely. Hope this helps others looking to solve this issue.