I assume you have a written agreement with this company. What did they (and you) undertake to do? Search engine optimisation is a bit of a black art and you may find that they claim that they have done things that would not be obvious to a layman. There may also be no obligation on them to optimise your site by a particular date. Rather than assuming that you can walk away for material breach of contract, I would write to them setting out clearly why you think they are guilty of nonperformance and give them the opportunity to respond. A nuisance, I know, but if you unilaterally decide to breach the contract you may find that they will sue you!