To finish the other side of what Marcus brought up...if you DO have a contract and it states they need to provide something in 3 weeks and they actually meet all the specification you request, then you are bound to live up to that contract. However, it doesn't mean that you cannot amend the contract. Approach them and tell them you want them to halt work, say you are not happy with the product they've produced this far and want to compensate them for the work they've done. If you do not have very specific design specs and you think they'll be able to say they accomplished it, then you should pay them 2/3rds of the work they've done. If you have specific requirements that you think they'll fail on, then you might be able to negotiate it to a lower value.
You have to think about it like this, even if they provide ugly stitched work, if they meet the guidelines of the contract then they've done what you've asked of them and they are due their money...regardless of what you were hoping for. You will have to chalk it up as a learning lesson on the importance of contract specifications.
In regards to the negotiation, they probably still have the right to say no and provide you with a working product for the full value, but just make the point that they are struggling to meet the deadline and if they fail you will pay them nothing.
P.S. I am not a contract lawyer, but I have done a lot of contract work, so this is information from my experience.
P.P.S. Don't leave yourself open to a lawsuit...not worth it.