That sounds like a bad idea. Foreign traffic exists, but is usually limited -- the Snapchat incident is a notable exception, but its worst harm was to servers in other countries from clients in the US anyway -- and no IP geolocation database is perfectly accurate. Indeed, in some ways, they're getting worse over time as desperate ISPs acquire IPv4 ranges from anywhere they can find them.
Trying to block such traffic will save you a small amount of bandwidth and harm a small portion of Pool users.
Are you sure they really weren't in North America? And even if they weren't, how harmful were they? A high traffic US server has hundreds of thousands or millions of clients, and receives thousands of queries per second. A small number of abusive clients are a disproportionate but still small percentage of total traffic.
Also, you can block or rate-limit them, with NTP's built-in rate-limiting facilities or in other ways, based on their behavior instead of their location.
In any case, the geo IP database is provided by a third party, and anyone can download it from them.