I hope you don't mind me jumping on the end of this conversation but I have joined Sketchfab with the aim of contacting a user with regards this very topic. I filed a usage violation with Sketchfab and Patreon over someone ripping a model from this site and then selling access to it on Patreon for $15 a shot as a game mod. As an artist, I find that quite shameful, so I tried to do something about it. Patreon said they will only act on a DMCA from the copyright owner but beyond posting a public comment, I haven't found a way to contact the owner yet. Sketchfab haven't replied yet, so I don't know what their stance is.
Is there a PM system, or is that something that is reserved for people with a higher status than site newcomer, or even just standard user?
Just to offer my background, I am a member of the modding community where this person uploads models (usually they use models ripped from other sources or blatant personal license abuse). I have been a professional game developer for over 30 years as an artist/designer/programmer, so consider an artist's creation to be worthy of protection. But unlike a crime the police respond to, a third party report carries no weight of any kind with copyright theft, so it needs the cooperation of the copyright owner. And that's not always easy to get.
I apologise if I have violated any forum rules in posting this, I am just trying to find the most direct way to the owner, to see if they are willing to act against this user.