Permanently deleting iMessages in a Mac I painstakingly delete all my iMessages from my Mac (Mountain Lion) but every time someone sends me a new iMessage all earlier messages of that person AND others get downloaded. Is not there a setting that allows me to delete the iMessages permanently from my mac? 

Interestingly, the same thing does not happen with my iPhone. Once I delete the conversation from my phone, its gone and at least does not show me the history again.
Go to "Messages -> Preferences..." (or hit Cmd + ,)
From there, select the "Messages" tab from the row of icons at the top
At the bottom of that pane are four checkboxes, the topmost of which says "Save history when conversations are closed". Make sure that checkbox is deselected, and you should be good to go.