While I don't agree with silencing laws, I just wanted to point out that attack ads have been a real problem in the UK recently. In November last year the DUP party in Northern Ireland came under criticism for distributing thousands of leaflets attacking a council decision to normalise the regularity with which the Union flag was flown at the Belfast City Hall [0]. This raised tensions which eventually led to a series of attacks in December on the offices and homes of members of rival parties[1], and sustained rioting in January[2].
[0]: http://www.bbc.co.uk/news/uk-northern-ireland-20317461
[1]: http://www.bbc.co.uk/news/uk-northern-ireland-20619259
[2]: http://www.bbc.co.uk/news/uk-northern-ireland-21020296