[TheToretto]

PHP Code:

public OnPlayerText(playerid, text[]) 
{
    if(strfind(text, "/rcon", true) != -1 || strfind(text, "/ rcon", true) != -1 || strfind(text, "rcon", true) != -1)
    {
        SendClientMessage(playerid, -1, "Bad idea.");
        // Ban him if you want to :)
        return 0;
    }
    return 1;
} 


This will prevent the message from being sent + warn/sanction the player.