Online advertising is the most critical revenue stream for many Internet companies. However, showing ads on websites comes with a price tag. Since website contents and third-party ads are blended together, third-party ads may compete with the publisher contents, delaying or even breaking the rendering of first-party contents. In addition, dynamically including scripts from ad networks all over the world may introduce buggy scripts that slow down page loads and even freeze the browser. The resulting poor usability problems lead to bad user experience and lower profits. The problems caused by such resource abusing ads are originated from two root causes: First, content publishers have no control over third-party ads. Second, publishers cannot differentiate resource consumed by ads from that consumed by their own contents. To address these challenges, we propose an effective technique, AdJust, that allows publishers to specify constraints on events associated with third-party ads (e.g., URL requests, HTML element creations, and timers), so that they can mitigate user experience degradations and enforce consistent ads experience to all users. We report on a series of experiments over the Alexa top 200 news websites. The results point to the efficacy of our proposed techniques: AdJust effectively mitigated degradations that freeze web browsers (on 36 websites), reduced the load time of publisher contents (on 61 websites), prioritized publisher contents (on 166 websites) and ensured consistent rendering orders among top ads (on 68 websites).