Roku doesn't block channel or media access. That is done by the channel creators and/or the media content providers. For the two channels that I know for certain are created by Roku, the Roku Channel is US only (although they might have added Canada, not certain). This is due I'm sure to the licensing agreements they have with the media content providers. They can only allow access to the locations agreed upon with the providers. The other channel is the Roku Media Player. It supports media playback over DLNA or for devices with a USB port a directly connected storage device. It has no geo-restrictions, as it doesn't provide any content itself.
If there are no Roku channels that offer services in Brazil, it wouldn't make much sense for them to sell and service the players in that country. The media providers must offer content before you can use a player like Roku there.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.