Keep calling, even while you are there - I am one of those people who actually cancel when we change our plans. On our Feb trip, we decided at noon that a 6:45 dinner at Ohana was not going to work out.
Tired Grandparents wanted a light meal and quiet drink at POFQ, while the revved teenager and I wanted to "powertrip" the MK.
I simply called from my cell phone and said we wanted to give up our table. The CM was astounded and asked if I was really sure about this
I assured him and said that we thought it was polite to open up space that we weren't going to use. He thanked me profusely and then laughed saying that someone would probably grab that ressie within the hour
I have also cancelled at our hotel concierge and at the restaurant podiums, anywhere from the days before to within an hour or two. It is so frustrating to see people turned away when tables are empty and it seems that the ADRs that they are being held for are "no-shows"

, so it makes me feel like I am spreading the magic

if I am courteous and cancel what I won't be using.
Good Luck!!!