I had the same question, I'm using v3.26. Thanks for the pointer. Here's more details I discovered for those interested in this topic:
Go to the "Content Settings" tab in the back end.
Scroll down to the "Ids, Sections and Categories criteria" section. There are three ways to exclude content from comments: by article ID, by section, and by category.
1) Exclude/Include item Ids: A text field in which you enter item IDs separated with "," and no spaces allowed.
2) Exclude/Include sections: A list box that allows multi-select. The selected items designate content sections for which no comments will be accepted.
3) Exclude/Include categories: A list box that allows multi-select. Selected items in this list designate content categories for which no comments will be accepted.
If you're new to making multiple selections in a list box, note that it's easy to mis-click and lose your previous selections! So don't forget to be careful when clicking (or CTRL-clicking) if you're trying to extend or change prior saved selections. Also, any selections you had previously saved will show up as highlighted in the list box.