Are you marking them as spam or are you "blocking" them.
If it is the latter that is not the way to deal with spam.
There is a limit of 600 on how many blocked email address you can have. If you want to keep blocking email addresses you will need to delete some of the email addresses already in the blocked email folder. You can do this by going into the account settings.
This facility is best suited to legitimate email addresses that you no longer want to get through. Spammers do not use legitimate email addresses and constantly change them so blocking them is ineffective.
Where as marking spam as spam will move it to the spam folder and eventually the BT servers will catch up and identify the emails as spam and stop them from even getting to your inbox.