I can't tell that there is a way to automate the closure of a Job posting when the pool fills. If there is a way to make this happen please advise how to make that happen. If not, it would be a very cool enhancement to Applicant tracking.
Yes, this is possible using the Begin Hire page.
In order to fill a pool, you must use the Begin Hire button. When you use the Begin Hire button, the automations on that page will close the posting if the pool has been satisfied. The only time it wouldn't is if you unchecked the boxes under the Hire Actions section that "Set job posting status to filled."