Privacy in My Spaces

I have Messenger 8.5 and in "My Spaces profile", my full name (first and
second) is showing, but when I try to edit my profile, and remove my last
name, the page returns with a message in red saying saying;
"Last name required"

But there is also listed beneath the last name window;

"Allow everyone to see my last name so people on Windows Live can connect
with me"

You have the option to click the box beside this message, which I have left
empty.

There is also this message:

"If you don't share your last name with everyone, it will be shared only
with people who can see your contact info".

I only want my contacts to see my full name. So how can I change this in my
profile to do so please?