How can I delete a channel? (old but gold)

Frequently Asked Questions End User Help
2

mmctl can do that:

# mmctl --local channel delete --help
Permanently delete some channels.
Permanently deletes one or multiple channels along with all related information including posts from the database.

Usage:
  mmctl channel delete [channels] [flags]

Examples:
  channel delete myteam:mychannel

Flags:
      --confirm   Confirm you really want to delete the channel and a DB backup has been performed.
  -h, --help      help for delete

Global Flags:
      --config string                path to the configuration file (default "$XDG_CONFIG_HOME/mmctl/config")
      --disable-pager                disables paged output
      --insecure-sha1-intermediate   allows to use insecure TLS protocols, such as SHA-1
      --insecure-tls-version         allows to use TLS versions 1.0 and 1.1
      --json                         the output format will be in json format
      --local                        allows communicating with the server through a unix socket
      --quiet                        prevent mmctl to generate output for the commands
      --strict                       will only run commands if the mmctl version matches the server one
      --suppress-warnings            disables printing warning messages