To delete a channel, click on the settings icon to the right of the channel name:
Then select "delete channel":
IMPORTANT NOTE: When you delete a channel all guides are removed from the channel - not deleted permanently. However, if you have not added guides to a separate channel, only the owner (creator) of the guides will have access to the guide in the "My Guides" section of the application. So ensure you add guides to new channels if you want to continue to have access to them in a team account.