If its really a human depending on what forum software you are running most have a mod or feature where you can "soft ban" them. The only one that can see their posts is them. :)

If they don't realize it they simply give up and go away, even if they stay it hardly matters because they are only posting to themselves. The other benefit is you're not escalating the situation because these people get off on the attention, if there is no attention they have no reason to do it.

It may take a while, but sooner or later, the penny will drop that they cannot beat you. Banning by email address may be more effective than IP - again, it takes them time to create a new one. And every decent forum forces email verification, of course!

If the set up allows, it's often useful to restrict the 'rights' of new members - the honest ones will not worry, but it could prevent a serial joiner from getting the benefits before being recognised and deleted.

There's another forum I hang out at where troublemakers come and go. It's been difficult for the moderators to figure out if it's the same people or not.