@B-RY-89 You make this sound so complicated. I only know dA's specifics through hearsay on NG, but legally speaking they have to have a DMCA/Copyright-claim link where you can tell them to remove stuff. (The site behind it should look something like this: https://www.newgrounds.com/wiki/help-information/dmca-notice )
If they refuse, you get to offer them to see you in court and pay your lawyer's bill. If they uploader keeps claiming that this is their original work, the previous sentence applies to them instead - or their legal guardians. :3
Yatsufusa
Either click the DMCA-link or write an email to their support.
B-RY-89
My only hope is to email them, which even then, that'll complicate things knowing how they are.
The real account I used to have (most recent name for it was B-RY-N) I've long since deactivated, since I was serious about leaving that site.
Unsurprisingly, this impersonation proceeds to happen not long after I've left the site.