Hi,
I looked into this, and unfortunately I don't have an immediate solution for you but I agree we should have some way to give individual control over those links for showing and hiding, perhaps we can add some display settings in the next release for this. I will also check if maybe yahoo has changed the format of their sharing url and fix that if so.
Those buttons are kind of old school compared to the addthis widget though. Note that you don't really have to create an add this account, it will still work if you put a bogus value for the account (it just can't be empty), the only benefit of the account is to be able to view sharing stats, the sharing still works without a real account.
Thanks,
Joe