Obama issued an executive order sanctioning Dmitry Rogozin and any organization he is involved with. Given that ULA were buying rocket engines from an organization that Dmitry Rogozin is involved with, they would be breaching the executive order by doing so.
Now this doesn't the contact has been stopped yet, as ULA have stated that they already have a 2 year stock of the engines and the injunction doesn't stop any sale pre-injunction. But if no other rulings come up as to why the contract should be stopped, they will still need to figure out a way to fulfill the launches that they do not have engine stock for.