Way of the Wicked Book 1 - Advice for a DM (SPOILER ALERT)

I've got a bit of a situation on my hands. When the party signed Cardinal Thorn's contract, two of them deliberately signed it with misspelled and incorrect names. At the time, I did not notice it when I looked at the contract because I was distracted by a third PC who was refusing to sign it(I had fun with Tiadora beating him into submission, glad to feed his hatred of Thorn already). I was completely unaware of it as they only told me after the session. I would have personally preferred a warning from them, but that is beside the point. How exactly should I handle this?

I feel that they probably couldn't easily fool Thorn like this as he's a fairly high level competent priest to a god of contracts. But at the same time I don't want to just immediately jump on them and punish them for this, as frankly I feel it's a pretty creative way to get around the contract. A second thought I had is, how would Asmodeus himself feel about this? I mean the contract is also binding the PC's to Asmodeus. So in effect they're also cheating, or at least trying to cheat, Asmodeus. Wouldn't this make him punish them?

I haven't read all the books entirely but I feel this could have some pretty bad consequences for the two PC's who refused to sign later on. Should I just have Thorn punish them severely and force them to sign it during their training they are about to undertake? Or should I leave it and have Asmodeus begin sending bad omens their way, for instance blood appearing on walls and writing out their names. Perhaps as a reminder that they've made the terrible mistake of trying to cheat the Prince of Darkness. As they may be able to cheat Thorn but they can never cheat Asmodeus. And how can I prevent them from betraying Thorn early and being slaughtered?

Advice? What impact can I expect on the adventure? There are so many ways I could handle this.