I will be happy to tell you how to remove that tab but what about the members page, do you allow guests to see the members page, if so then they can see who is online already by the online indicator next to the name.
A simpler solution IMO would be to just not allow guests to see the member link. I think you are hurting your self by having that page set to guest view. That would be the best option for you so that you do not have to change the core code.