This instance is not used for posting. It is private in the sense that people only use the infrastructure to access posts on a different server. Like a proxy server. You are only seeing this because federated servers are visible, and because admins may want to know who they are connected to.
If you see anything illegal on this server, then it was probably created somewhere else in the fediverse. So the text was copied to this server, but image/media is hosted somewhere else. Essentially, there is nothing I can do, and you have to contact the originating server to delete the content in the entire fediverse, including on this server.