Deactivating a user does not mean that the license will not be included in the billing. To avoid getting billed for an inactive user with an assigned license, make sure you unassign the license from the User page then deduct the license from the Subscriptions > License tab. Deducted licenses from inactive users will apply on the next billing cycle.
To deactivate a user:
From the top Navigation, click the User Menu > Control Panel > Users
Scroll down to locate the User List, use the search field as needed.
Locate the user > click on the dropdown option under the Status column > Click Inactive
Click View.
You will redirected to the User Details page.
Click Edit.
Locate the Assigned Subscription License field
Toggle the switch off on the assigned license.
Click Update.