But I agree that's fair to expect from companies. Yes, they have potentially hundreds of applicants, but writing "We're sorry to inform that you have not been selected to interview" probably takes less than a minute, so spending less than an hour rejecting every applicant seems in line with the time I'd expect each candidate to spend preparing and submitting their application. Plus you can always automate a list of emails to send rejection messages to...
It's also a nice way to differentiate the Who's hiring? from all other job boards out there
Maybe make the "flag" feature work for users to "report" non-responsive employers. On repeat offenses, reach out to them saying they've been repeatedly reported? Just brainstorming
Perhaps the monthly postings should be handled via ycombinator.com/jobs and the thread here is just a dump of this month's new openings with links to applications there but not direct posts by companies?