Similar to how junk mail is shoved into your letterbox, spam email is information that is useless to you. This is information that you never wanted and is that integral part of the internet, which you cannot escape no matter what. Thus, BT, like a couple other email services provides its users with spam filters. These filters work towards removing any unneeded emails even before they reach your inbox.
Sometimes some reputable companies like a utility company send you marketing emails which you may or may not want. It may not have been information that you asked for, but it is legitimate and cannot be considered as spam. These emails tend to come with an “unsubscribe” button after clicking on which you will no longer receive any emails from them.
There are certain types of scammers who send you email that look like genuine marketing emails. This is a technique used to trick you into revealing personal information like you bank details or you BT ID password and email. This is called phishing. Any serious harm can be avoided by not clicking on any link that is sent by even a slightly suspiciously email.
There is no other way of dealing with this, because even though it might contain disturbing things like pornography or other disturbing images, it is still not illegal. If you see a spam with things that are illegal, like child pornography, then you can contact Internet Watch Foundation as BT is a member and then report it to them.
By being extremely careful, you can avoid or at least reduce the chances of getting spam and other unwanted emails. However, there are still chances that you might get spam emails. This is because spammers sometimes guess valid email ids by spoof. All they do is take up potential usernames and add it to a username. This is called spoofing.