Locking a billing period basically do import lock. It will stop coming usage data into billing period rather blocking other manual functionalities such as generating transaction, generating invoices, sending invoices etc.
1. Click on Billing Periods in the main menu, list of billing periods displays in a table
2. Click on the billing period you want to lock. It will open billing period invoices page
3. Click on Status button to lock the billing period. Status will change to "Locked"
Note: To unlock the billing period click again on the Status button. Status will change to "Open".