Actually, you can't. If somebody in your team stops working for your company, you'll want to archive his user account rather than delete it. Here's why.
When a user gets archived in your account, he'll no longer be able to access your account, but all his tracked time and data (like tasks, to-dos, comments, etc.) will remain available for your records. Deleting a user would result in the deletion of all his data, specially his tracked hours, and thus affect your project and task times and reports. That's the reason users cannot be deleted.
Fair billing policy
Please note that you can archive and reactivate users anytime according to your needs. User fees are automatically prorated and you only pay for those users who are active and have access to your account during the current billing period.
Here's how to archive and reactivate users.
In you'd like to permanently delete a user and all his data from your account, please get in touch with us and we'll help you out. But keep in mind that this action cannot be undone.