The ability to directly blacklist an IP or email address from the spool page is not possible. I can see the advantages of being able to blacklist directly from the spool page. Therefore, I have added this idea to our feature requests list for further consideration by the dev. team. I am changing this from a Question to an Idea thread.
We are planning on significantly changing the layout and grouping abilities for the spool messages in the next major release of SmarterMail to make system admin's jobs much easier. This looks like a good candidate to be included.