Like many other players, I am becoming increasingly frustrated and annoyed with the many whispers I get each day from gold sellers or people telling me that Blizzards coming out with "a new mounts" and crap like that. The vast majority of these whispers come from players under level 5. I'd like an addon that whenever someone under level 5 whispers me, they are automatically responded to with a message telling them to go away, I'm not interested. Is such a thing possible?

I use Automatic Goblin Therapist, though I don't believe it's available on Curse.  It's good for a laugh...

SpamSentry, you can set the level to be ignored.  I have mine set at 60.

BadBoy: Anti Spam Automatic Reporter is wonderful for this. Not only does it block gold spammers but it also has a plugin called Badboy_Levels that blocks anyone at or below your preset range (for example you could set it for lvl 5s and all lvl 5s and below would be unable to whisper you).

If you're looking for more of an over-all blocker Give Me Peace is also nice. GMP can be used if you just want some peace and quiet during rep farming or for blocking anyone not on your friends list or in your guild.