You can block a single email address (among other things) using Content Filtering in the SmarterMail interface. Here are instructions on how to do so:
1. Log in to SmarterMail.
2. Click the "Settings" link on the left-hand side navigation menu (it is an icon that looks like gears).
3. Click the "Filtering" folder.
4. Click "Content Filtering".
5. In the Content Filtering window on the right hand side, click the "New" button at the top.
6. On the next page, under "From Address", click the checkbox next to "From specific addresses".
7. Click "Next".
8. On the next page, leave the radio button next to "AND - All criteria must be met" selected. In the text box under "From specific addresses (one per line)", enter the email address(es) you want to block. Click "Next".
9. On the next page, enter what name you want to give this Content Filtering rule in the "Name" field.
10. Decide what you want done with the filtered message by checking the box next to one of the listed options.
11. Click "Save".