You have to stop them from registering in the first place, thats how you right them. You need to fight them on several levels, there is no one battlefield you have to fight them on several fronts. You need to change some "out of the box" text that they search for to find you in the first place, then you need to block well known IP and email addresses, then you need a good captcha in order to make it much harder for them.
Then if they do get past all that you need to block them at server level if you can.