You can delete an entire channel at once, if you have permission to do so.
Who can delete a channel?
To delete a channel in Slite you must either be:
Admin of your Slite team
Admin of the channel you are trying to delete
How do I delete the channel?
In the left sidebar, click on the three dot menu beside the channel name that you'd like to delete. Select "Settings". You'll have a "Delete this channel" button on that popup menu.
Deleting a channel will send all the notes in that channel to the Trash, where you can restore them (or permanently delete them.)