This is a legitimate concern. I've been meaning to address it myself. Adding an address to 'blocked senders' is way too easy for beginning users who are just learning SmarterMail. We had one user who selected their entire Inbox, chose Actions->Block Sender and then called us ask why she wasn't receiving any email.
Suggestion #1) Get rid of the option to block senders as it's not needed in today's environment with advanced spam filtering. Blocking an address is useless because spammers don't re-use old addresses. Users should mark as Junk if they want the system to learn. Furthermore, we don't want users to be blocking legitimate email -- unsubscribe! If this feature is truly needed for some obscure reason, the user can go into Content Filter and create a rule.
Suggestion #2) If #1 is not an option, explain it better. When user chooses block sender, there's a pop-up. In the pop-up, list the address(es) being blocked so they can confirm. Explain that all future messages from this user will be discarded. Tell them that a rule will be added to the Content Filter in case they want to revert. Give them an OK and Cancel like all good dialog boxes have so they can back out. And finally, it would be nice if the default was to move these messages to Deleted Items or Junk instead of /dev/null.