Deleting the file with the trash can doesn't really deletes the file as the link is still reachable. I've seen an FAQ post saying there is no solution aside from asking for help on a public forum (here), is that really the best prismic can do?
Hey Marcus,
Please submit a request through our support portal:
https://prismic.atlassian.net/servicedesk/customer/portals
We'll take care of the data removal for you 
Sam