Click on the arrow at the top right hand corner of an unwanted ad and choose 'I don't want to see this' and you'll be given the option to hide all ads from that particular organisation.
You can reject specific ads in your news feed too. The equivalent mobile setting is limit ad tracking. The opt-out is per-browser and per-device, so if you use multiple browsers you'll need to opt out in each one (and opt out again any time you clear your cookies). You can opt out of targeted ads but if you clear cookies you'll need to opt out again