Is there such a thing as a video blocker?

I would've thought an ad blocker would've had an option to not allow videos to autoplay, seeing as most of the ads that people find really annoying use video.

I use NoScript, which is a Firefox extension that's intended to stop malicious JavaScript from running. It also blocks Flash videos, replacing them with a "click to play" box. (I'm not sure if it blocks HTML5 videos, though I don't know how many sites are using those so far.)

It uses a site-based whitelist, so once you've decided that you trust a particular site not to try to hack you or annoy you, you can whitelist it, and that site's scripts and videos will always be allowed after that. You can also temporarily whitelist a site, which lasts until you revoke it or until you close the browser.

The main disadvantage of NoScript is that it'll take you a while to figure out which sites to whitelist, especially because a lot of sites nowadays bring in libraries and frameworks from other sites, and tend not to work very well if those are blocked. But once you've got over that hurdle, the web is a much more peaceful place :-)

Opera web browser lets you disable flash and javascript until you reenable it.

you can do it in Chrome but it is harder...