BT isn't sending you spam. Someone who has hacked into your contacts list is.
Once someone has accessed your contacts list, they'll be able to send out spam (not only to you but to others as well) that looks as if it's come from the people on that list. They'll also be able to send out spam which apparently comes from your own address. It's too late to do anything about it, other than to warn everyone on your contacts list that it's likely to happen. (An AB member, Anaxcrosswords, had his email contacts list hacked several years ago. My address was on that list and every few months I receive an email which appears to come from him but which actually only has a dodgy-looking link inside it. It's too late for him to be able to do anything about it).