As far as i can tell the only way to report somebody is to
a) warn as many of the actual artists of theft (a great help is https://www.duplichecker.com/reverse-image-search.php)
b) individually report each piece of stolen art (look for the three dots under the stolen arts right side)
if there is a way to report the entire account all at once so i can avoid tedious work i haven't found it
I personally do A and B then c) block the thief so they don't clutter up my recommendations and so on.