It has sent emails to people around the world it has detected are using software tools to watch HBO Now shows.
HBO said it took the action because it only has the right to broadcast shows in the US and to people living in the country.
Email messages were sent to people in Canada, the UK, Germany and Australia, reported tech news site Torrent Freak.
HBO Now was introduced as a way for people to watch HBO's programmes without the need to have a subscription to a television or cable service. HBO Now costs $14.99 (Â£9.50) a month.
However, many people living in other nations have subscribed to HBO Now and used a variety of tools, including virtual private networks (VPNs), to get around the restrictions designed to stop people outside the US seeing popular shows such as Game of Thrones.
In some countries, the cost of an HBO Now subscription was far lower than the price of buying a service from a native cable TV provider. The Sydney Morning Herald pointed out that watching HD versions of Game of Thrones in Australia via means other than HBO Now would cost more than 660 Australian dollars (Â£342).
Earlier this week, HBO started sending emails to many people who use region-unlocking tricks and tools. The messages warned them that they would be cut off on 21 April if they did not contact HBO and satisfy it that they were eligible to watch the service.
Those who did not contact HBO would be cut off without further notice, said the company.
Earlier this year, Netflix started to take action against some VPN users and changed how its Android app worked in a bid to thwart some of those using software tools to defeat region locks.
Netflix boss Reed Hastings said earlier this week that VPN users were "less bad" than pirates because they were at least paying for the service. His comments came during press interviews discussing the company's financial results.
Pay TV service HBO is threatening to cut off paying customers of its Now service if they are outside the US.