Is there a way to ensure that a resource does not exist? I’d like to delete the AWS default VPC for new accounts.

I don't think so. I can think of a few threads of ideas we've had that if implemented could probably support this use case, but not sure if this is doable today. Feel free to raise an issue at https://github.com/pulumi/pulumi/issues explaining your use case and we could give it a proper think over.