How do I stop Yahoo Sync?
I was curious about this myself and started by looking in Activity Monitor, found that "launchd" was the service that kicked off the Yahoo Contacts Sync.  After some googl'ing I found out that custom scripts for launchd get executed from the ~(user)/Library/LaunchAgents folder.  In there, you'll find the culprit -- at least, I did.  Now I dunno if I should simply delete them (since I don't care about syncing contacts, I don't use the Mail.app) or if I can just disable it by tweaking the plist file.  Hope that helps!

EDIT: I discovered that going into the AddressBook.app preferences and unchecking the accounts listed caused the plist files to be removed.  If you check them again, they reappear.  Works for me!  :)