It is true that you can now choose what scores you can send but some schools can choose to override the choice and have the College Board send them all scores in spite of your choice. I believe at least some of the Ivies choose to override, so you can't rely on that to hide your scores.
Schools will interpret a cancellation neutrally. They won't know why you canceled -- perhaps you got sick in the middle of the exam and had to leave. And they all know that a single sitting of any exam can go bad for any number of admissions-irrelevant reasons. So if you really feel uncomfortable about the exam then go ahead.
However, before you do, consider the following points:
-- The essay question is just one portion of the the Writing section. You get a base score from the multiple-choice part of that section and then your score on the essay section (maximum of 12) is used to adjust the base score. Roughly if you get a 10 on the essay the base score comes remains unchanged, a 12 will raise the score (a little, perhaps 20-30 points), an 8 will depress your score a little (about 20-30 points), a 6 a little more, etc. I can't say how much a 0 will decrease your score, but it could still be a worthwhile score. And you are unlikely to get a zero - they will still give you points for coherence and structure even if you are off-topic.
-- Moreover, sitting the exam twice will not harm you in the least; the large majority of applicants will have taken the exam at least twice. So, if you do get an unacceptable writing score you can, without negative consequence, have another go. (It is possible to look bad if you sit the test too many times--you will look test-obsessed--but that threshold starts at about four.)
-- In fact most schools, including the Ivies, will consider only the best of your scores on each section of multiple exam sittings ('superscoring'), so by canceling you might be throwing away two excellent scores on the other sections.
So my advise would be to let it stand and, if necessary, try again later, unless you have reason to think your other sections also will fall far short of representing your actual ability.