You have to code this in your form.
Edit your form's atttribute, in the onLoad attribute set : ca_fd.js.onLoad()
Add the following in the form's script. You will need to change the content of the fieldsToDisable array.
onLoad : function()
{
var fieldsToDisable = ['txt_test1','txt_test2']; //Edit this line. List every item, using it's _id, that you want to disable.
if(_.request.status==800)
{
ca_fdDisableFields(ca_fd.formId,fieldsToDisable );
}
}