The Bat has to receive a message before it can do anything with it, so, it cannot prevent that messages from that person will be downloaded from the mail server to your PC (if you do not want to receive the messages at all, maybe your provider has a spam filter that you can configure to filter that person's messages).

Having said that, The Bat can delete them as soon as they arrive, for example like so:

If you still have a message from that sender, look it up in the message list and right-click on the sender's name. Pick 'create filter' from the context menu that has popped up.

The fields in this dialog should be filled-in as follows:  (note: the first three should already be filled-in correctly by The Bat)

Name: (Give the filter a distinctive name)
Detect by: [v] Sender: <sender's email address>
Use for: Incoming Mail
Actions: [v] Move to: Trash (use Browse button on the right to pick the Trash folder)

Finalize by pressing OK. From then on, all incoming messages from that person should be moved automatically to the trash folder. If not, go to 'Account | Sorting Office/Filters' to review and edit the filter.

I don't see 'block' anywhere, so I can't help you with that. (maybe because I"m still using an older version).