This mod from Mallyuk allows you to block whispers from people who are not members of your guild or on your friends list.
Blocks whispers from other players who are neither:
-in your guild
-on your friends list
A basic addon for when you want some privacy. Couldn't find an addon that did just this, so I made my own. My first attempt at LUA...
-Added a simple, movable 2-button frame to show the current status, and allow easy toggling on/off. It can be hidden with "/blocker nogui"
-Settings now saved per-character
-Will no longer send an auto-reply whisper when a message is blocked if it is less than 3 seconds since it sent the last one. This is to avoid creating a loop if the other party has an auto-response (addon, afk, DND etc)
-Some minor cosmetic changes
NOTE: On first installation, or after upgrading from 1.0.0, all settings will default to OFF. See readme.txt or type /blocker to see a list of commands.