How can I block someone?

Hello,

I am sorry for this generic question title, but I really don’t know how to sum up this.

Here is my problem : I have my own Openfire server (3.8.0 on Gentoo), with this server, I am using my gtalk account, and someone that I don’t know wants to register to me (don’t if it is the good way to describe this).

I am using Psi as client, and every time I am loggind to the server, a popup ask me if I want to accept this contact, I click reject, Psi show my this contact as offline (maybe because I have rejected it) and in a 'Not in list" list.

The problem, is that is keeps asking me that, like if the reject action didn’t do anything.

How can I completly ban this contact ?

Do I have to install a plugin for that ?

Help me please