Cookie to prevent access of unwanted user

[Mark Hewitt]

We have a user who is consistently accessing the board to cause trouble and to make threats on the life of one of my moderators (the police are involved).

However I was thinking about a general way to get rid of someone permanently that doesn't involve email or IP addresses, both of which can change.

Is there a way that I can write a hack so when someone who I know is a person I don't want logs into the forums a cookie (or other such thing) is placed on their computer to idenitify who they are and then thus denied access to the forums.

Critically this cookie should not be deleted when a user logs out of the forums. So even if he logs out and registers under a different name he will still be denied access.

I know that he can get rid of the cookie manually if he wants to. But I don't think he has the knowledge for that sort of thing.

Your thoughts appreciated.

Thanks