The system will bill you at the end of your billing period for the number of active users that you had during the billing period.  This includes users who were deactivated during the billing period, but were active at some point during the billing period.  

To deactivate a user, go to Employees>Click Employee>Personal Details.

Untick the "Active" box, then click Save on the bottom of the page.

To ensure that you are not billed for the user, the user will need to be deactivated during the entirety of your billing period.

Note: You cannot deactivate a user if they are still an approver in an approval group.