How to prevent Mail.app from automatically updating Address Book entries?

Hi,

Before iCloud was launched I kept my iPhone's Contacts separate to OS X's Address Book. Now with the iCloud Contacts sync I wanted to merge the address books and stumbled onto a problem. For some people I prefer to keep their entries in Adress Book as First Names only. I use it e.g. for my family and friends. Sometimes in the First Name's field I put their nicknames istead - just prefer it this way. There's no need for surnames as I am able to identify anyway.

Now, with iCloud active every time I answer to my family's emails sent from business adressess where they use full names, Mail.app automatically updates my Address Book with their surnames. This then automatically updates Contacts on my iPhone and when someone from that group calls me I see Nickname LastName calling and it's obviously just wrong. I do not want Mail to automatically update my contacts. How do I stop it?

Michal

PS. I found this discussion, but no solution there. "Use Last Names" is not an answer for me.