I think it depends what the arrangement is. If you hire someone to create deliverable X with defined specs for $Y then if they take the contract they need to provide deliverable X to spec no matter how long it takes them; or you don't pay.
Its up to the contracted not to accept a job where the specs are inadequate. If the job is going to be big enough that its worth while maybe you provide some good customer service and help write clear specs; that everyone can understand and agree to.
On the other hand lots of contracts are written block of time. In which case I think you are obligated to pay for additional hours if that is what is needed to make something work or decide not to continue, but you still pay for the time used so far. Now you still may have issues with the quality and quantity of work falling below industry norms. If that happens you maybe don't hire that person again, and if its really really out-of-line that is what courts are for.