Must admit not heard of BT Yahoo 'disposable' accounts, but easier enough to create yourself using yahoo.
I tend to use yahoo as a disposable email address for any subscription lists or anything I want to sign up to, keeping my main email account (a bit cleaner of poteintial spam) Funny enough was just setting one up for my uncle yesterday for just this purpose.
Probably the best bet is to create a account via here click here= (nice 6mb of storage!) Then use it for as long as you need to, simply creating a new one if the need arrises. To keep the spam down from the start don't sign up to any yahoo list, or offers.