In this situation, the regular account closure process applies. You can find more details in the following Help Center article:
Note: We are aware that some embassies and consulates are currently closed due to the COVID-19 situation. If it is not possible to receive the document called “Sperrfreigabe” or the visa rejection letter in person, we can also close the account with an official confirmation email from the embassy or consulate. However, this must be sent to us directly from the embassy. No additional documents are required.