By default, comments are allowed on all Sketchfab models. However, if you would like to turn off comments on a particular model, you can do so in Model Properties.
Disallowing off model comments
To turn off comments, simply open Model Properties, by clicking Edit Properties on the model page, then click the Allow comments toggle.
Disallowing model comments in organizations
Models inside an organization account can also have comments disabled as described above if the model is set to a "Sketchfab" visibility (Public, Private, or Password-protected). However, this will only turn off comments on the public model page. Internal comments will still be available to organization members on the organization model page. If a model is set to "Organization Project Only" visibility, there is no public model page, so comments cannot be disabled.