Another method is to implement one account per IP address/location within a single game.

Sure, the most ambitious cheaters could still skirt this with VPN, but there are also additional methods on Colonist side to prevent or detect VPN usage as well.

Colonist could require a location upon Signup, then only allow that player to play if his/her IP is from that location. Permitting several location changes per year, or something similar.

Colonist can record player's IP address upon logon. If history of IP logins varies over X amount of times within Y period of time, the player's account gets flagged for review. VPN users will typically use multiple location servers which is not representative of a real user's player location habits.