I do the same and it's immensely useful!

I find it's also a great indicator of how well that vendor protects my info... if there's a sudden surge of spam to an address only that vendor knows, there's a chance they leaked/sold my info and I know it's time to change passwords and/or reassess my relationship with the vendor.

If keyboard shortcuts are enabled, '!' is mapped to "Report as spam". I don't recall whether the status popover that appears at the bottom of the UI has an "Undo" link, though...

You have to:

1. click on the email from the inbox

2. click on the three-dot hamburger menu on the far right of the sender line

3. click on the 'Block "Evil Sender"' entry in the popup menu

4. confirm that you do in fact want to stop receiving spam from the spammer by clicking the "Block" button

5. Click the "Move to spam" button in the dialog box that appears