Approach #1: Open positions that are no longer available.
The employer pays the fee and gets 30 days of public presence. No more, no less. The position may be relevant for days and weeks after the posting went offline. It would be quite expensive for the employer to pay for another month, so they rarely choose to pay again. As a result, you won't see the position even if the employer still considers new applications.
Approach #2: Closed positions that appear as vacant.
The employer pays the fee and gets not only his 30 days of fame but many more. The position may be irrelevant for months, but visitors still see the listing. You don't know for sure what is the current status of this job. A few years ago, I found myself browsing jobs that are six months old. I spent around 10 minutes before I discovered this, thought 'what the hell?' and closed the page. When a person is excited or desperate about getting a new job, or just not attentive enough, this can easily result in a shameful waste of time.
Unfortunately, that's not an accident. These pages were created this way intentionally.
When I was building the website, I didn't want to give an administrator the exclusive power to decide which jobs are still relevant, and which are not. Of course website owner can do this but that would not be sufficient in a long run. The picture with hundreds of jobs that we need to verify and extend or close depending on their status doesn't look promising. The good news is that we don't have to. Luckily for all, the company owners or an HR person who is in charge of the recruitment process can decide whether they need to stop accepting applications or not.
Even if they fail to update their listings, any job seeker that finds out that the position should have been unpublished can report it, and we'll make it disappear.
Now you know how we deal with this problem. Toy Theorem is almost self-sufficient system, an environment where visitors support the balance of interests for everyone involved. Simple, efficient, and fits perfectly in the overall "employer - job seeker" equilibrium.