To "close" your billing account:
Go to the Cloud Platform Console https://console.cloud.google.com/
- Hamburger menu: select Billing.
- Select the billing account name. (Do this twice.)
- Click "Close billing account."
- Type "close" to confirm.
By deleting GCP you mean deleting the running project? If that is the scenario then you can detach all the attached resources and then delete all the services that you are using. This way you will not be charged for anything. Because GCP is a pay as you use Cloud Service. Hope this helps!