New UI: Mark a card as ‘known’
To stop reviewing a word you already know, just change the card's status from 'learning' to 'known'. At the end of your card session, click on the word you want to stop repeating...
You'll be directed to a page detailing the card's history. Click on the gear icon at the top right corner of your tab.
Select 'Mark as Known' in the pop-up window to archive this card. There you go, you don't have to review this card for the next 90 days. The card will pop back up once to verify that you really have mastered the vocabulary, but you'll skip all the review sessions in between.