Stiftung ear enforces fees that have not been paid after one reminder. The reminders are always sent to the main contact. Enforcement is carried out by customs.
Advance payment, or respectively, surety
If there are constant arrears in payment, or respectively, recurring reminders to a debitor owing fees, it is possible that the fulfilment of a service applied for will be made dependent on advance payment, or respectively, a surety according to § 15 BGebG.
You find the legal basis here.