Self-service sign-up: Method by which a user signs up for a cloud service and has an identity automatically created for them in Azure AD based on their email domain.
Azure AD cmdlet Set-MsolCompanySettings could help you to prevent creating user accounts with parameters:
AllowEmailVerifiedUsers (users can join the tenant by email validation)-->when is TRUE.
AllowAdHocSubscriptions (controls the ability for users to perform self-service sign-up) e.g. Set-MsolCompanySettings -AllowEmailVerifiedUsers $false -AllowAdHocSubscriptions $false Reference:
https://docs.microsoft.com/en-us/azure/active-directory/enterprise-users/directory-self-service- signup