This is a fairly interest subject by (and in) itself. There are tons of programs out there that make it appear as if information is coming from one computer when it is not. The biggest and best known is TOR but this is a bouncing server, meaning it is bouncing one signal to a computer to another computer and so on. This means the actual sender can never be traced. On the other end they have software that appears a single is coming from one computer (and one computer only) when it actual is not. Thereby, this selective method works with the general public but not someone with some knowledge of what products exits now days.
The only reason I know this (is my night job is a hacker 'just kidding') is we had to securitize our site. Actually CoStar has the best system to stop this type behavior by submitting a code key to the user direct (within minutes of use) but it cost about $150 per user. It is fairly cost prohibitive to most unless you are charging CoStar prices. There are other methods but this is one of the best. The second best is (and there are arguments about this like everything) a user name, password, (various firewall softwares) and CAPTCHAs codes. At least site owner will know if two users are on at the same time and if it appears to be coming from the same computer (or different computers or as you pointed out from outside the country). However, be aware, it is day time in the US and night time in India and the Philippians so in the example I gave it is very hard to trace. They can make it all looks to be coming from the same computer. Hacking is nearly impossible to stop. That is why you continually see these large companies targeted.
Oddly enough my cousin works for the system used by CoStar.