Billing periods can be marked as ACTIVE if they are still valid for you or INACTIVE if you do not want to use them. Inactive periods are defined as those in which there has not been a single exercise in the last six months or is only used within the collection bins. This setting is used to show billing periods more clearly in reports for courses or groups, and also when transferring registration.
To change the billing period from active to inactive you need to do it in the Settings tab in the left menu. Then click on Billing Periods for a list of all billing periods.
Select the period you want to deactivate and click on the pen icon for editing.
Then you need to click the Active billing period option. This will make the billing period inactive and will not appear in the list of billing periods within Settings, Courses, Groups, and Transfer Registration.
If you want to show inactive billing periods in the reports, you need to click Show Inactive Periods under Settings.
More about billing groups can be found in the manual How to work with billing periods.