It was going much too fast to stop (given the available propellant). Also, that would have extended the transit time significantly. On the upside, it's sounding like the Kuiper belt is a really interesting place, so we'll get to see much more!

It's moving too fast for it to stop. It's not [currently] possible to ship enough fuel for it to stop by using rockets. And there is no good atmosphere for it to use to stop (plus we don't have enough information about the atmosphere to design anything yet).

So it will just do a very short flyby, then spend the next year or two sending the data back to Earth. After that it's not decided, but probably some Kuiper belt object like you suggest.