"Spam"? Or "Junk"? Thunderbird's own built-in nuisance detector refers to bad stuff as Junk, not Spam. It learns by looking at the frequencies at which words occur and correlating those with the user's classification of the message as either Junk or Not Junk. So the Junk Controls in Thunderbird are not a blacklist and won't automatically block messages from that sender. If the message has somehow been categorised as "Spam" then that will be done by some program or service external to Thunderbird, such as the anti spam service offered by your own security software or by your email provider or ISP. To fix that you need to go to the relevant security software or the website where (maybe) you can de-register that sender.