Yes, open the details page by tapping the information icon in the upper right hand corner.
Next edit the channel or note permissions.
Select if you want only group admins or everyone to be able to post.
If you select only group admins, other users will not be able to post to this channel, but will be able to view the posts. This is helpful for admins to post notices and announcements.
Other users will see the following message in the input bar.