I found that in:

kamctl db show presentity

Is the presence information of the users, but how can I errase such entry? of a user?

And how can I define only one entry in that database by user? I use this code, when I receive a publish but this adds a new entry every time.

if(is_method("PUBLISH"))
    {
        if($hdr(Sender)!= NULL)
            handle_publish("$hdr(Sender)");
        else
            handle_publish();
        t_release();
    }