I can't find a solution on how to do that, but there is a deactivate account but I want to remove the account permanently. How can I do that?
On Year 2017
This answer is provided by Slack itself, Admins or even Owner can't delete the users permanently because of this reason:
According to Slack, the profile information of the user can now be deleted, the article is here:
For full details, visit the link provided.