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If you are already supposedly in a contract - just don't pay them, and wait for them to take you to court.
If they actually do take you to court, extremely unlikely in my opinion, say that you weren't made aware of the auto-renewal clause at the time and that the service they provided did not bring you the traffic they promised and as such was not value for money.
Legal threats is one thing - if you call their bluff, its up to them to carry out those threats, which is probably going to cost them more than they will retrieve.
IanTurner is correct. Force them to come after you. Just do not completely ignore it as there are other things in contract law such as "summary judgments" which can go against you if you do not reply to a lawsuit!