Spam hack

Hey.
I seem to have gotten some kind of spamhack on my server.
There is .xml file automatically made a couple of times a week that Yoast seo warns me about.
So i can delete it via Yoast or ftp. But it only comes back a couple of days later, and it is screwing with my indexing.
I have looked at the file and sometimes there only thing there is the letter 1, other times there is what looks like spam.
Anyone knows how to remove that?

Fredrik