[DragonicRek]

Send them corrupted files using any of the below services.

http://www.corruptmyfile.com/
https://corrupt-a-file.net/
https://pinetools.com/corrupt-a-file

No doubt they'll continue to request the files, so keep sending them In different file formats (this gives the Impression as though you're making every effort to comply and resolve the matter), be apologetic and adamant that they're working fine on your end.

EDIT: In the process of my post, @Auftot has suggested something similar In the first line.

What are the differences between using that website and just corrupting by editing as notepad and deleting the characters?