When you deactivate a user, you will have the option to remove that unused seat in the Billing Overview page.

• If you remove this unused seat, the changes will take effect in your billing the beginning of your next billing cycle. If you fill the seats with new users before this date, the seats won't be removed.
• If you do not remove this unused seat, the seat will still exist in your account and billing remains unchanged.

If you want to remove seats from your account after deactivating users in the past, go to your billing section and select Manage seats.