Shave it all off... and THEN let the chin grow normally, don't shave it in front and let the under-the-jaw-line part grow... that looks bad. Unless that's how it grows and you didn't trim it?
actually I trimmed the neckbeard forest underneath my chin down to where it was in the pictures, having trouble getting the hair follicles stimulated to grow all the way on my face..I had hair below my lower lip but I had shaved that when I shaved the mustache.
Ah, facial hair. It's always disappointing when you realize you can't grow what you want.
Me, I once tried to grow a full beard when I was cast as a King in a shakespeare play. But the sides of my cheeks only grow about two hairs per square inch. I ended up having to stick with this rather disappointing goatee.