He’s the only Glenn Grujic in the USA (NYC area), and he has an active Facebook account with activity from as recently as yesterday. Shouldn’t be too hard to track down. Legally your only recourse is small claims court, unless your talking thousands of dollars - then criminal court.
If ignored, and when there is a belligerent transaction or theft with an individual, I usually start messaging their friends, family, and workplace via Facebook or forums to stoke the issue since the offender usually doesn’t want them to know and they might rectify things to get you to stop. Watch your step though. We sometimes delete posts like this since there is no way for the forum to know who is right or wrong, but I’ve always thought it was good for collectors (with limited funds) to know who / what to avoid.