Whenever you see a tweet you didn't want to see, pick a word directly from the tweet and add it to your block list right then. Don't worry about false positives too much because it's just Twitter, missing a tweet or two isn't the end of the world, and also I often only block words "from people you don't follow" so the blocks don't apply to direct tweets from people I know. Pretty soon you'll have a great block list and a clean feed.