If you are on the BTMail email system and not the BTYahoo system you should log onto your email account via webmail (On a browser) and go to "Contacts" then tick the small box at the top of your contacts list next to the search box. This will put a tick beside all the email contacts. Once you have done that go to the "More" button at the top of the list and select "Delete".
Having never don this myself, you may find that depending on the number of contacts you have you may need to do this a few times as the tick box may only become active for the number of contacts that are visible on your screen's web page.
If you are using a mobile device you will need to use the "full version" of the browser and not the mobile version so that you see the tick box or better still use a grown up computer or laptop.
