Using the Sync app (on Android anyway), you can hit the settings button while browsing posts, and under settings there's an option right there to hide read posts.
On the web, if you click on your username in the top right and go to settings, there is an option "show read posts". By default I think this is turned on - uncheck the box to turn it off. For a while, this wasn't working, but it seems to have been fixed along the way.
Congratulations and good work!