I don't understand. You stated that Thunderbird marks it as junk. What else would you have it do?

Once they mark it as junk, why do they keep letting emails from the same email address show up in my inbox? If they have identified an email address that is sending junk, they should delete all emails from that address.

Thunderbird, on its own, looks at each incoming email with a fresh view. If you know an email address is unwanted, then you should create a filter.

If they know that email address is junk, then why can't they delete the email from that address each time they receive it?

Your suggestion about creating a filter is what I used to do, however, when I try to create a filter now, there is no place to click OK or Save or Submit. I can fill out information they ask for, but again there is no place to tell it you are want to submit the information. This happened after my last upgrade of Thunderbird.