Please follow these steps to delete your Azure AD B2C tenant:
Delete all the User flows (policies) in your Azure AD B2C tenant.
Delete all the Applications you registered in your Azure AD B2C tenant.
Next, sign in to the Azure portal as the Subscription Administrator. Use the same work or school account or the same Microsoft account that you used to sign up for Azure.
Switch to the Azure AD B2C tenant you want to delete.
Select Azure Active Directory on the left-hand menu.
Under Manage, select Users.
Select each user in turn (exclude the Subscription Administrator user you are currently signed in as). Select Delete at the bottom of the page and select YES when prompted.
Under Manage, select App registrations (or App registrations (Legacy)).
Select View all applications
Select the application named b2c-extensions-app, select Delete, and then select Yes when prompted.
Under Manage, select User settings.
If present, under LinkedIn account connections, select No, then select Save.
Under Manage, select Properties
Under Access management for Azure resources, select Yes, and then select Save.
Sign out of the Azure portal and then sign back in to refresh your access.
Select Azure Active Directory on the left-hand menu.
On the Overview page, select Delete directory. Follow the on-screen instructions to complete the process.
https://docs.microsoft.com/en-us/answers/questions/511/how-do-i-delete-my-azure-ad-b2c-tenant.html