Firstly, you made the same proposial 4 days ago. Secondly, I think there is no suitable way to stop it. Thirdly, I think it would be a bad idea. If you want to play against someone in your family, the easiest way is to play as a guest.

And what does it matter anyway?

Why go making such a fringe suggestion (AGAIN!) and not have a reason? 

> I like to surgest that players with a ranking are not
> allowed to play as guests.

I disagree.
I sign on as a guest at home on an older computer with slow DSL.
When I am work (new computer, fast internet) then I sign in.

c²

Well well

I have a proposal that solves this problem. We need an option to play UNRANKED. This would give this game that much more interest and effort, because anyone could ask for a game where rating is not afflicted. 

Go to unrated room...