Make sure the app is enabled (the toggle button in the top left corner should be green) and check your rules for typos and the correct matching type.
Stil not working? Does the song happen to have multiple artists? Spotify only returns the main artist, so if the artist you despise is the secondary artist on this song, it might slip through. I'm looking at ways around this.
Something else? Send me the rule you've created (type, match, keywords and offending song) and I'll look into it.