I disagree. Some people may be considered judgment-proof, meaning they have no assets to be sued for (O.J. Simpson, for example). However if you win a judgment against someone who has assets but refuses to pay, a court could order wage garnishments and even asset seizure. Basically the courts can make you pay by taking from you. (At least, this is how it works in the USA.)

Of course cases with citizens of different nationality are different and much more difficult to work with, even with a paper contract. Both governments must be willing to hold their citizen up to the agreement in the contract. If I have a contract (printed, signed, and notarized) between myself and the deposed king of Nigeria, it would be hard for me to take the ex-king to court.