Open Automated Rules in the side menu and click New Rule. Give it a name (for example, Pause if CPA greater than US$50), choose the Ad Accounts (or leave All accounts) and, under Apply rule to, select Active Campaigns. Under Action, choose Pause. Under Conditions, click Add condition, select the metric (for example, Cost per Purchase or CPA), the Greater than operator and the threshold value, then confirm. Finally, set the Calculation Period (for example, Today) and the Check Frequency, and click Create Rule. The rule is active from the moment you create it, and you get a notification whenever it pauses something.