[erminpr0]

pawn Code:

new bool: Banned[MAX_PLAYERS char];
CMD:ban(playerid, params[])
{
    if(!isnull(params)) Banned[params] = true;
    return 1;
}

OnPlayerConnect(playerid)
{
    if(Banned[playerid])
    {
        SendClientMessage(playerid, -1, " You are banned ! ");
        return Kick(playerid);
    }
}

Make your own ban command, this is example, if you don't understand I'll make it later !