I had this problem not long ago and it's returned again. I seem to be unable to block terms. I click the block icon, and it turns red, but next time I check on it, it's gray again, and the word continues to show up in quizzes. This happens for all terms, not just specific words.
It was also happening on quizzes. I kept trying to block one term during a quiz, and it kept showing up again. I can verify it's fixed for dictionary pages, and it seems to be fixed on quizzes as well. Thanks!