There is no way to prevent non-US buyers with a shipping address (which do not have to be in their country of registration) in the US from buying from you. You can prevent those whose primary shipping address is in a location you don't ship to (to limit where you ship to the US, choose domestic shipping and not international shipping and opt out of the eBay International Shipping program* in your selling settings, and be aware "US" includes AK, HI, PR and several US possessions such as USVI, Guam--those can be specially excluded from your shipping zone) from buying or committing to buy by setting a Buyer Requirement blocking those with a primary shipping address (again, doesn't have to be in their country of registration and can be quickly changed as needed to meet such buyer requirements).
Anyone who does not have a shipping address in a location you ship to cannot pay for the item. You can use Buy It Now only (no auctions or Best Offers) and make the BIN "Immediate Payment Required" to prevent someone who doesn't have such a shipping address from buying.
You are not under any obligation to ship anywhere you didn't agree to ship. The buyer can only pay if s/he gives a shipping address in a location you've set up to ship to. If s/he does (many international buyers use forwarding services or friends or family in the US) your obligation is to deliver the item as described to that address, any forwarding from there voids the Money Back Guarantee. If s/he doesn't you can cancel for non-payment after 4 full days.
*ETA: "You" don't ship to other countries if you are enrolled in the EIS (the default is that you are, whether it is available for a particular listing if you are depends on whether it meets the criteria for inclusion in the program), YOU ship to an eBay's contractor facility (I think one near Chicago and one in California) who ship it on to the buyer, and you and eBay/contractor split the s/h the buyer pays--you get the same as shipping to a buyer in that facility's location).
