You can close your entire account in AWS Billing: https://console.aws.amazon.com/billing/home?#/account
Or if you just want to disable your “Free-Tier” services that has charges, view them here:
https://console.aws.amazon.com/billing/home#/freetier
Then open your EC2 dashboard – and cancel those services:
https://us-west-2.console.aws.amazon.com/ec2
For example:
Stop running instances, delete volumes, remove elastic IPs, etc.
Otherwise, I recommend sending an email to webservices@amazon.com from the email you used to signup with their service.