I used SpamMeNot, but I don't think its being updated any longer, and the only other alternative I've found is BadBoy: spam blocker/level filter. Only problem is it has a auto report feature that you can't turn off. So I;m just looking for a new addon that just blocks whispers from lower level players/gold sellers, etc. If anyone knows of one, please let me know.
I haven't tried it personally, but there's a plugin for BadBoy that claims to do that. More info here (http://www.wowace.com/addons/badboy_levels/).
I use all the Badboy addons and have for a year now. Wouldnt use anything else.
http://www.wowace.com/addons/bad-boy/ (http://www.wowace.com/addons/bad-boy/)
http://www.wowace.com/addons/badboy_ccleaner/ (http://www.wowace.com/addons/badboy_ccleaner/)
http://www.wowace.com/addons/badboy_levels/ (http://www.wowace.com/addons/badboy_levels/)
http://www.wowace.com/addons/badboy_guilded/ (http://www.wowace.com/addons/badboy_guilded/)
and if you dont trust it and want to watch what it filters: http://www.wowace.com/addons/badboy_history/ (http://www.wowace.com/addons/badboy_history/)