You are right, we should allow deleting multiple channels.
Short of that, you can try listing TV channels in a smart list. Just select "Media Type" to be "TV". All your channels are listed in the smart list. To make sure your smart list is setup correctly, you can check the "Filename" for each entry. They have filename extension "tvlb". The filenames all begin with "TV Channel" and ends with "([tuner type])", like
"TV Channel xxxxxxxxxx (ATSC).tvlb"
"TV Channel xxxxxxxxxx (Set Top Box).tvlb"
You can select multiple entries in that view and delete them.