Most profiles (apart from 'viewer') have comments enabled by default. This means that users comment on dashboards or favorites that have been shared with them (as well as any they created themselves) and see other people's comments. Administrators can change this, either via a default profile affecting multiple users, or in an individual user's profile.
To change this setting go to Administration and either:
select Profiles to change a default profile, or
select an individual user to change their profile setting and go to the Profiles tab in the user maintenance form.
Under the section heading 'Collaboration' untick or tick 'Comments', as required, and save the change.