How to opt out of an ad on Yahoo Answers screen?

Use Adblock Plus to hide it.

I used to allow ads as well with the mindset that as Yahoo were providing a site which was free to use that the least I could do is keep ads visible.

However I reached my limit last year when they started with noises, with filling entire pages, with in-line question ads and then finally when Yahoo allowed ads with malware. After that I installed adblockplus on my main browser and to be honest I haven't looked back.

If you have installed that already and it doesn't work (make sure it was the plus version and not just adblock) then also install this one https://addons.mozilla.org/en-US/firefox/addon/flashblock/ and it should stop the Flash player as well.