You can permanently remove a person from the organization using their Person Profile located in the People screen. If a user is removed their account will be marked as deactivated and they will be removed from the organization. However, all their content and messages will still be available in all groups and direct messages.
User Deactivated state.
Step 1. If necessary, make sure you're in the Manage Organization area.
Step 2. Click the People panel to display all the people in the organization.
Step 3. Click the person’s name or Profile Image to display their Person Profile screen.
Step 4. Click the Delete User option located in the bottom-right corner.
Delete User option.
Step 5. Click the Delete User option again from the confirmation panel to accept the process and permanently remove the user.
Delete User confirmation panel.