The quickest way to delete all messages in a slack channel is to use slack-cleaner. You’ll need an API token first so log into slack and then open up this link: https://api.slack.com/custom-integrations/legacy-tokens
Scroll down to “token legacy generator” and you should see some green buttons.
With this token in hand, install slack-cleaner. On mac it is:
> pip install slack-cleaner
Then just run this command
> slack-cleaner --token <TOKEN> --message --channel general --user "*"
You might be rate limited so some deletes may fail. If so, just up arrow and run again.
If you want to delete or technically deactivate users and you are on the free plan, then you are out of luck.
Trying: https://slack.com/api/users.admin.setInactive with the correct parameters will result in: