You need to understand this, yes AWS offers you free service usage. But that comes with limit. You are chargeable each time you exceed those limits. Now about restricting your account:
If you have Account Root User, you can’t restrict access to services.
However, you may create an IAM user with permissions that apply only to one region, and then login as that IAM user.
You may want to keep an eye on the billing page in the console to be aware of charges and the amount of the Free Usage Tier remaining. Set billing alarms to receive a notification when your account usage exceeds a given amount.