I am doing some keyword research and its apparent that two potential competitors are using PBNs for some of their posts.
In fact, for a specific keyword, the top two posts ALL of their links, except for one, are PBN garbage...

Is it possible and feasible to report a competitor for using what a PBN is?

Don't bother to report a competitor's PBN because its no use, just focus on your website on how to provide a page for the target users.