Click the ▸ arrow next to the person’s current team role.
Click Deactivate Account.
Once the team member’s account has been deactivated, they will no longer be able to access the Slack team and cannot reactivate their own account. Any apps or custom integrations added by the team member will also be deactivated.
Keep in mind: To deactivate an Admin's account, you must be an Owner. Similarly, an Owner's account can only be deactivated by the Primary Owner of the team.
Note: You can't delete a team member entirely — you can only deactivate their account. This is to make sure your team's message history makes sense when you read it back.
Message history for deactivated accounts
Deactivating a team member’s account does not delete their messages and files, which remain archived and searchable. Any messages or files that need removal can be deleted before deactivation.
Direct message history between you and former team members can be accessed from the Deactivated Accounts tab on the Directory page.
Tip: The search modifierin: followed by the person's username, will help you find your direct message history with a former team member.