Key Management Service Generating Cost

KMS keys are used for encryption across many services. EBS or S3 would be obvious culprits but if you were using encryption with any of the services that you were trying, the charges are likely coming from there based on requests.

In the KMS console look for 2 types of keys as seen below – AWS Managed and Customer Managed Keys. The AWS Managed ones will point you to any services that might be using encryption. If the service is not accessing the key it will not drive cost. The Customer Managed keys to have a monthly cost per key and can drive cost based on usage. (Note: Check the KMS console in each region where you tested to look for keys.)

Additionally, you can go into the Billing and Cost Management console and look at your current bill. You should be able to find a summary of the charges by region like below that will show cost for keys and number of requests to use keys.

Hope this helps you track down the usage!

How can I ensure that all services have been canceled and there will be no future charges?

Just to add to what @iBehr said, even if you have some usage you can't account for (and you should still follow @iBehr's advice for how to track down and remove these) as long as it is fewer than 20,000 requests per month then this will fall within Free Tier https://aws.amazon.com/kms/pricing/

Free tier

AWS KMS provides a free tier of 20,000 requests/month

This is forever, not just for the first year.