The answer is probably painfully obvious but I'm stuck.
I just discovered Channels (w/ HDHomerun 4K Flex) and was having a look around the WebUI (macOS) and marked several channels as "favorites" but can't see how to remove a favorite.
Thanks.
Dvr web ui > settings > basic setup > click the gear on the hdhr source > manage line up > click heart to unfavorite
In the webUI, just click the 
icon again to unmark it as a favorite.
In the app, just select and hold a channel in On Now and choose Manage Channel.
You can quickly hide or favorite channels from a source by going to Settings > Manage Sources. Pick your source and you'll see all of its channels. Click them to cycle through Hidden, Favorited, and Unfavorted.
As a new fan of Channels, you might want to learn about Channel Collections: