There are various plugins to your web browser you can install. They work a bit differently depending on the program, and I'm not sure what browser you're using. Some of these, I haven't personally tested and I'm not sure they're even the best ones for each browser. But they should do the job.
Once installed, you usually get an icon somewhere in your browser (in my Firefox, it's near the top right-hand corner), and it lets me quickly add a site to the "disabled" list so ads will show up. On some sites, it is necessary to do or content won't load properly. But by default, it disables ads.