How are they supposed to know your the real owner of the "original EPS" file? Sure, they have a JPG, but for all they know, you could have "stolen" that EPS file you're using as proof.
Imagine what it would be like if anyone could just point a finger, yell "copyright infringer" and get the persons account deleted....
No, never I only just point a finger.

Normally, when I contact agencies in this situations, I show links from my portfolios where the original illustrations are.
So they can see, that my files are online for years and been sold 100x or 1000x.
Also they can see, that this is my style, cause there are much more illustrations like this and you can see that my portfolio was natural growing over the years.
Mostly this and that the theafs have not only stolen my pictures in their portfolio, which you can check often only in minutes is reason enough to close the account of the theafs.
Sometimes the agencies asked me for more informations and I gave them the information they need.
But mostly the informations I gave them in the first mail are enough.
Also in this case with alamy. I send them links to my portfolios, where they clearly can see, that this is my work.
So, when this is not enough I can send them more informations, which show clearly who is the theaf.
In this case the two pictures are some of my bestsellers and been sold more than 10.000x .
(and also illegal downloaded thousands times.)
I think agencies and contributors have to work together against theafs.
It's NOT only my work.
So, the answer of alamy is really unacceptable.