Find and delete a malicious script in my site

My website keeps getting suspended by the host due to sending out spam emails. Each time they are able to tell me where and the name of the offending script. They unsuspend my account so I can go seek and destroy. But over night it re spawns just like in a video game. Puts the offending file someth where within my directory and it continues to send out spam emails. So there's a script somewhere within my site which creates the second easy to find script which I keep deleting. Its a word press site. If you win this job we will have a few hours to accomplish the task. As I have to call the host, unsuspend my website, let you in, then you fix. I would also like a detailed explanation of what you did as part of this. Simply deleting the easy to find script is not satisfactory I do that daily. Its the underlying script which I need proof of a kill. :)