Short answer: unless you hold the copyright to the works in question, there is nothing you can do. Sending a DCMA to a work that you don’t have the copyright for will invalidate the DCMA.
Even then, it generally is a waste of time trying to play whack-a-mole with the pirates because they outnumber the author and are not too concerned about obeying IP laws anyway.
Putting the shoe on the other foot, there are plenty of stories here that are basically doing the same thing, only to artists instead of authors. Putting up a disclaimer that you don’t own the work/found it on Pinterest or Google/full ownership goes to the artist, does not absolve the author of using an image for a cover without the permission of the one who holds the rights to that work, i.e. the artist.