The same is happening to me on my main account. I'm getting over 20 a day that are not filtered out into the spam folder and hundreds into my spam folder.
I found a way of using the rules to make sure the Inbox contained only senders you want, it has kept my Inbox clear of spam, but it may not suit everyone! If you look in the discussion topic "Spam filter" it's on Page 10.
(I would put a link to this post but I am not sure how!
How would BT know its spam, you may have actually won that lottery you never entered
All you can do is to use the extensive filtering options available, similar to what I suggest on this page.
Plus other suggestions that have been posted.
If it is a BT email sub account you can delete it your self. If it is your primary email address it is a bit more difficult and would require BT doing that and changing your account details etc.
Would it not be easier just to stop the old email address from forwarding and then stop using it. That way, if it is your primary email address you can still use the user name to access BT services such as BTWifi and you can occasionally check the account to see if there is anything important being sent to it.
This has started happening to me again too - it died down and spam was being put in my spam folder and that was fine - now it's in my inbox - sorry BT/Yahoo, but it's from multiple sources, which means ... someone your end HAS changed something, so can you please (re)fix it asap ...
PS I do NOT want to have to set up filtering - it was fine as it was, until someone changed something your end!!!