Since .com domains are assumed to be targeting the United States by default, you don't need to do anything to target American searchers beyond having a site that is in English and is linked to from other English websites. There are a few reasons you'd ban visitors based on IP/location, but none of them have to do with improving rankings.
These include:
- You have different a different site for international locales (you would want a 301 redirect, not an IP ban)
- You want to prevent forum/blog spam (Cloudflare would be a much better alternative than doing this manually)
- Legal reasons people from a specific location shouldn't be accessing your site or page - the only legitimate reason I can think of to ban large blocks of IPs