You can easily define what elements are allowed to have comments made on. By default, comments are enabled for all elements.
As you can see from the above screenshot, you'll have access to all element groups used on your site. These are your entry sections, category groups, asset sources, user groups, and custom elements from other plugins.
To control commenting on the element-level, create a new fieldtype using the Comment Options fieldtype. This contains a single lightswitch field, denoting whether you wish to enable of disable comments on the element. Make sure you add it to the elements custom fields'.
Available for Craft 2 for free, Craft 3 coming soon for $49.