First, go to Your Study Sets and select the Studied tab. Then, open the More menu next to the title of the set you want to hide and select Remove.
Once you confirm that you want to remove a set by selecting Hide this set, you'll be good to go!
How do I hide sets from my Latest feed on my mobile device?
You can only hide sets from Latest Activity from the Quizlet website. To do this on a mobile device, you can log in to your Quizlet account from your device's browser and follow the above instructions.