Log onto your BTMail account using a browser, not an email client/app, and then click on your username at the right hand side. Select "Settings" then go to the left hand side and select "Mail". Scroll down and click on "Blocked Senders". Enter the address and click save.
Or high light the email in your list of emails and right click the select "block sender".
You will find that if it is a spammer's email you are wasting your time blocking the address because spammers change the address frequently to avoid being blocked.
