If you have enough candidates that it's too much work to reject people, you likely have enough candidates to merit at least a simple templating and candidate management app or plugin. It is absolutely trivial to one-click reject groups of people.

Really?

While I’d love personalized feedback, all I really want is for your Applicant Tracking System to fire off a short email from noreply@company.com, bcc’ing everyone other than the person getting an offer.

   Subject: Your Application to $company

   I regret to inform you that $position at $company has been filled by another candidate. We appreciate your interest in working with us.
 
   [Add other platitudes, info about reapplying or whether you’ll be considered for other jobs here…if you want].
   —- $Name or HR department

If you’re too pressed for time to do that…maybe you should be hiring someone who can.