Hi @Camiel,
It seems like you're facing recurring charges for a deleted Notification Hub, we can see that you have already taken few steps to resolve this. To address the issue:
Go to Azure Cost Management + Billing to review the individual charges. Determine the subscription, resource group, and region of the charges. Quickstart: Start using Cost analysis
Some Azure services have soft deleting capability, which keeps deleted resources for some time to avoid accidental deletion. While Notification Hubs do not have native support for soft delete, it may be that related resources, including storage accounts consumed by the Notification Hub, have soft delete enabled, resulting in ongoing charges. Soft delete for Azure Storage Blobs generally available
Make sure that the Microsoft.NotificationHubs resource provider is registered properly. A misregistered provider could lead to inconsistencies in resource management and billing. You can verify this by going to: Azure Portal > Subscription > Resource Providers. Search for Microsoft.NotificationHub and check its status. Azure resource providers and types
If the resource no longer exists and charges persist, you can reach out Azure billing and subscription management team by creating a billing support request. Billing support request via Azure portal is free for all Azure customers and doesn't require a paid support plan. Please select 'Billing' under 'issue type' while creating the support request.
https://azure.microsoft.com/en-in/support/create-ticket
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