Why am I still being charged for Lightsail?

Hello.

How to stop being charged every month for a service I don't use and stoped a few months ago?

Could you please share the exact details of the invoice?
You need to see what items in Lightsail you're being billed for.
You can view your invoice by following the steps in the document below.
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/getting-viewing-bill.html

How to contact client service to get this charges stop and get a refund?

If you have a dispute with your AWS charges, I recommend that you open a case with AWS Support under "Account and billing".
https://docs.aws.amazon.com/awssupport/latest/user/case-management.html
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/checklistforunwantedcharges.html

For questions about your AWS bills or to appeal your charges, contact AWS Support to address your inquiries immediately. To get help, see Getting help with AWS Billing. To understand your bills page contents, see Using the Bills page to understand your monthly charges and invoice.

Hello,

There are two possible reasons:

1. Please check some others dependencies related to Lightsail like static IP,DB instances you may miss out.
2. Check others Regions there may have some other Lightsail you may have created.

First check cost explorer and invoices for further investigation.

Hi,

We noticed that a static IP resource was previously present in your account, which was removed after your post. As a result, you should no longer incur charges for any Lightsail resources associated with your account. However, if you continue to experience any billing-related issues or concerns, please do not hesitate to contact our customer support team. We will be happy to assist you further.

Thank you