No easy way.
Have to go to Home Trade region and lower right side button to 0.
A pain in the a--e when you're exporting 45 of something that you want to stop.
(Like with UK or Prussia often in the beginning.)
By Home Trade region do you mean the Ministry of Industry and Commerce? I can't figure out from there how to make exports of a single, particular, such as steel, be 0.
Center the map on your capital and hit the T button.
Opens trade screen to enable trades from your country.
Prussia is actually spread across 3 trade regions but everything gets concentrated in the capital and then sent out from there.