Thunderbird's Junk Controls do not filter on sender. Most spam or junk doesn't come from the ostensible sender. Few spammers would use their own address and they wouldn't keep on using the same address. So by and large, identifying Junk mail by sender isn't awfully effective. What sort of messages are you getting that repeatedly come from the same sender? I see a significant amount of legitimate email traffic labelled as "spam" (by google, gmx, and so forth) and it seems to me that a lot of people become tired of newsletters, circulars and commercial messages and report them as spam, where in truth at some point they must have signed up for these messages, or at least submitted an email address. If these messages are addressed to you, and you alone, then in this situation, you should unsubscribe. If it's not possible or otherwise unattractive to unsubscribe, then you could and should set up a filter to deal with these messages automatically. OTOH if you continue to mark these messages as junk, they may eventually become recognized as Junk. But it is not a fast process, particularly if the messages have comparatively innocuous content.