It seems like they sold you a product at a 50% discount. Your argument seems to be that you didn't use the product therefore you'd like to return it. This is where the situation gets sticky. Just because you didn't use something doesn't mean to call return it or not pay for it in terms of a virtual service. They sold, and presumably made it available for use and it still could be available for use, so in there mind they have fulfilled the agreement. I'm sure at some point in your life you've paid for something you got no use out of, we've all done that, this is just a bigger than normal veersion of that.
Read over the contract and see what the cancelation clause is, and what the dispute clause is. It will be in there. If there's not, which I doubt, it will fall to your states law on purchases and disputes. If can show some kind of misrepresentation then your chances of getting out of it go up but you'll likely be spending money on a lawyer.
From am monetary business perspective they probably have a good case on there side. From a $200m company reputation standpoint they look like assholes... They need a reality check that you can't act like Oracle until you are Oracle. It would be better to cancel it and then try and do business in the future.