"Je préfererais pas" is valid (and matches the contraction of "I'd prefer not to"), although a bit less formal. Almost equivalent to "Je préfererais ne pas (le

"Je préfererais pas" is valid (and matches the contraction of "I'd prefer not to"), although a bit less formal. Almost equivalent to "Je préfererais ne pas (le faire)" is valid too in everyday speak. And slightly different from "Je ne préfererais pas".