Navigate to your 'My Media' page. You will see your media
arranged in a list. On the right side of the page, each entry will
have a pencil icon 'Edit' button. Select this to edit the media. You will
also find an edit button on any of page where you can view media
that you own. It will be below the video on the right, in the
'Actions' drop-down menu.
On the 'Options' tab, you can choose to 'Disable comments
for this media' and 'Close discussion (do not allow new comments)' Disabling comments will hide any previously entered comments
from the media. It does not permanently delete them.
Click the 'Save' button to save your changes.
For a channel you manage:
Navigate to your 'My Channels' page. Choose the channel you
would like to disable comments on and click its pencil icon 'Edit' link.
On the channel's 'Details' tab, uncheck the 'Enable comments
in channel' checkbox to disable comments in this channel.