There is no "you". Thunderbird is email client software running on your computer. Contact BT and see if you still have a working email account that you can log into. They should have a webmail interface where you can use a browser to check your account. Thunderbird does not use or create a Spam folder. Your provider most likely does. They should have a button to empty the folder somewhere. Maybe right click the folder and see if the option is there. You can always click one message in the folder to select it, then press control+a to select all and press the Delete key.