EDIT 2: The truth was somewhere in the middle. You can't send a takedown notice on a piece featuring your character that was uploaded by the artist. (Also, I'm editing this post 'cause it got shared a lot on its own, so I'd like anyone who stumbles across it in future to get the full picture.)
EDIT: Got my wires crossed a bit. Linking @GoopySpaceShark's reply, since Cohost doesn't make replies easily visible. The "proper" process is in fact sending a takedown request, but the DNP list is only availible to the artist of the piece; so it's either proactively asking artists to get a DNP when you commission them, or playing whack-a-mole.
The "proper" process is to contact every single artist who's drawn art of your characters, and have them submit a DNP (Do Not Post) for those pieces. Because e621 does not consider the risk of getting DMCAed for art made by a third party featuring your characters significant enough to allow character owners to put them under DNP. It also has the "benefit" of making it logistically painful and socially awkward to get art of your characters taken down.
