Mobistar to Proximus

In november 2009 I was co-erced to save money by signing up to a two year contract with Mobistar. In Nov 2011 they have somehow managed to create a 2 year extension to this contract without my signature nor known intervention. Now I want to END their ficticious extension contract, they reclame 150 euros costs ! !
Does anyone know how to get around these horible people, no one ever told me of cancellation costs beyond the original 2 year period. Many thanks for some serious advice. Richard2828.