Knockout.js validation is handy but it is not robust. You always have to create server side validation replica. In your case (as you use knockout.js) you are sending JSON data to server and back asynchronously, so you can make user think that he sees client side validation, but in fact it would be asynchronous server side validation.
Take a look at example here upida.cloudapp.net:8080/org.upida.example.knockout/order/create?clientId=1 This is a "Create Order" link. Try to click "save", and play with products. This example is done using upida library (there are spring mvc version and asp.net mvc of this library) from codeplex.