To prevent any placement or rate changes made after billing is generated, go to Organization Settings > Organization Preferences > click on the Settings tab and enable "Restrict placement and rate changes to administrators only before".
In the date field, enter the date that will end the restriction. For example, if you want to prevent any placement or rate changes being made for April 2020 billing (this includes all months prior to April as well), set the date for 5/1/2020. This will restrict changes being made before that date, but will allow changes for that date and beyond.
Enter in the billing contact, including name and contact information. Any worker who tries to make a placement, new case admission, or rate change prior to the date will be prohibited from doing so. All users with full administration or billing administration permissions will get the same warning, but will be able to proceed with any changes.