WordPress Security

Hi
I hope some one point me to a plugin to resolve problem I have.
I have two client sites and they have been swamped by HACKER(S) posting GARBAGE in to comment area of the blog or page( in the page no comments to begin with) any way no one can posted unless they registered and there in no new user registration, I have disallow ip address , nothing stop them .So here is a question do any one know plugin or a way to close this hole
Thanx