If someone leaves your organisation, you can revoke his/her user profile.
To do so, go to your 'Team' section and find the user's card. Click on the 'Settings' logo in the upper-right corner of the card and then select 'View profile'.
At the bottom of page, you'll find the 'Revoke' option and you'll be able to delete this user after entering your password.
Only admins and primary owners can revoke a user.