Depends on if you have booked a restaurant that has cancellation policies.
Most restaurants have no cancellation policy or penalty and you can cancel them at any time.
A few restaurants (the prepaid ones like CRT and dinner shows, plus any restaurant that requires a credit card hold - California Grill, Akershus, Victoria & Albert's, special meal packages like Fantasmic or Candlelight Processional, some special dining events like Lunch with an Imagineer) have a cancellation policy. The cancellation deadline varies by restaurant. At these restaurants if you don't cancel and don't show up you are out either your full prepayment or a per-person no-show fee.