This is ridiculous. Target the promotion to US-only.
Now, if your page is liked by folks in foreign countries then you still may get (organic!) foreign likes, but you shouldn't be getting any paid foreign likes.
Absolutely does not work on Facebook. Or, I can't figure it out.
Every time I run Facebook ads I target them to Raleigh, NC, USA only. Almost all the resulting clicks are foreign (according to the IPs I see in my analytics).
You can't target only people in the US for promoted posts. The only options are to promote to Facebook fans or to promote to Facebook fans and friends of fans. Many of those friends of fans have ended up coming from foreign countries in my experience for a local restaurant.
THIS. Click-through-rates, likes, and shares are going to vary dramatically from country to country. If you're running your ad to everyone in the world, the clicks will be SO much cheaper and you'll get served in countries where clicks are cheap and click-through-rates are high.
Now, if your page is liked by folks in foreign countries then you still may get (organic!) foreign likes, but you shouldn't be getting any paid foreign likes.
tldr: layer on US-only targeting