Only the restaurants with credit card guarantee requirements (CRT, Akerhaus, and the dinner shows, maybe some others) will bill you for no-shows, and even then, they'll bill the CC, not your plan balance.
As a courtesy to fellow guests, I would suggest calling Disney Dining and canceling the ADR if you aren't going to show, even if it's minutes beforehand, so that there's a chance a walk-up or a last-minute reservation will be able to accommodated.