It has been confirmed by BT that users are unable to switch of Spamguard on BT Yahoo accounts. In fact even although the user has used the option (if available) to switch off Spamguard it fails to do so without any indication that it has not done so. Accounts that had SpamGuard switched off, have had Spamguard switched on without the user being informed.
A consequence of this is that valid emails that SpamGuard thinks are spam are now being deleted without a user being informed. This is particularly true for users using an email client that does not automatically download emails form the Spam folder.
This change seems to have been implemented partly as a result of the recent spate of account hackings totally missing the point that mechanism used by the hackers is to send out emails that are not picked up by SpamGuard because they come from people that are likely to be in their address book.
BT Yahoo say that the change is to protect users from malicious emails while the effect is to make them more at risk.