looking to have traffic originating from any country other than the SA and Mexico blocked

If you are using AWS CloudFront Distribution service, you can configure Allow-list in Geographic restriction section to allow for SA and Mexico. And whatever origin is fronted by CloudFront, you will configure it to only allow connections via CloudFront and no direct-access to Origin ( for example ALB).

There are 3 ways to block traffic by Geo, these can be used for public facing endpoints such as Elastic Load Balancers:

1. Using CloudFront Distributions: https://repost.aws/knowledge-center/cloudfront-geo-restriction
2. Using AWS WAF: https://repost.aws/knowledge-center/waf-allow-block-country-geolocation
3. Using Route53: Not blocking but redirect users outside of the approved Geos to a different endpoint with either an error or no reply.

If these solutions are not applicable can you provide extra details about your use case, like the services you want to restrict, which will help people answer.