The only time I tried this I quickly gave up, because the poster I didn't want to see used the monthly quotes threads to karma boost. A malicious user that's intelligent can pretty much troll however they want and still have positive karma simply by copy/pasting quotes into the the quote threads.

This doesn't really apply to the topic at hand, but the quote threads are a serious karma problem, because they provide an effort-free way to generate huge amounts of karma.