If you have a "Windows" keyboard, the sequence Menu Key, J, B will add the
selected message to the Blocked Senders list.
If you don't have a Windows keyboard, Alt+A, J, B will do the same thing.
If you want something more direct, you can use Tools - Customize Commands -
Actions category, and drag the Add Sender to Blocked... command to an
existing toolbar. Assuming that the Back key is already displayed on a
toolbar, you'd either need to decommission its &B hotkey, or assign a
different hot key to the "Add Sender to &Blocked Senders List" command. For
example, you might change the whole named of that command to "&Junkify", and
then you'd be able to press Alt+J. ;-)
