We understand this is frustrating. It frustrates us, too. We're limited by the technology available to us. In the case of Twitch.tv ads, there are technical limitations a browser-based ad blocker such as AdBlock simply can't overcome.
Although AdBlock can't block the ads on Twitch.tv, we do have other suggestions that may help.
Ads don't appear on channels you're subscribed to
Subscribing to a broadcaster removes all advertising on their channel. (Thanks to gamingwithzee and blastoise186 for this tip!)
An Amazon Prime account also removes ads
AdBlock fan blastoise186 suggests this tip: If you have (or are willing to get) an Amazon Prime membership, you can connect your Amazon account to your Twitch account. With a "Twitch Prime" subscription, no advertising will appear anywhere on Twitch. (Twitch Prime also lets you subscribe for free to one channel each month for double coverage.) The streamer still gets revenue. It's just from your Amazon Prime membership, the benefits of which you're probably enjoying for other reasons, instead of from advertising. That's win-win in anybody's book!
Whether you subscribe to individual channels or have a Twitch Prime subscription, make sure you're logged in to your Twitch account to get the "no ads" benefit of having a subscription. You can use Twitch.tv's "Keep me logged in" setting to avoid having to log in every time.
Warning: DO NOT set this if you're on a public or shared device.
Another reason to log in is that as long as your subscription is active, your subscriber benefits are applied wherever you log in to Twitch―in any browser, on any device, including the desktop site, the mobile site, and the Android and iOS apps. (The apps also keep you logged in all the time.)
Try the Twitch 5 player for Chrome or Firefox
Update December 12, 2017:
The Twitch 5 player has been removed from the Chrome Web Store. The Firefox version, however, is still available. (Thanks to blastoise186 for the update!)
According to a tip from a user (shout out to ALG DZ) and verified by user reviews, the extension Twitch 5 strips pre-roll ads from live streams.
Get it for Chrome: Twitch 5
Get it for Firefox and browsers derived from Firefox: Twitch 5
(thanks to Discountoxygen for pointing us to the Firefox version!)
If anyone knows of an equivalent for other browsers, please let us know in the feedback for this article!
The HTML5 player can help block ads
This tip comes to us from another user (shout out to Pascal!). Make sure your browser uses the HTML5 video player, not Flash. This blocks the ads on Twitch for some viewers. Your mileage may vary.
Search for and remove the ad element manually
Thanks to Xxchaosxepsilonxx for this tip: When you see an ad in a Twitch stream, press F12 to open the browser's developer tools, then press Ctrl+F to open a search box. Enter "js-ad-overlay" (without the quotation marks). Select the <div id="js-ad-overlay"> element and press Delete. Again, your mileage may vary.