How do I prevent search terms from being copied between applications?
To give you an answer the best I can with the research I've done. This is a relatively new addition in the OS. If I am not mistaken since 10.8 and what happened is Apple changed the Helper daemon.

I do not know of a way to configure the helperd parameters. My recommendation is to give Apple feedback via http://apple.com/feedback/ and ask that they give the ability to toggle it or remove it all together.

Additionally ask the mad scientists here to include the functionality 
http://www.bresink.com/osx/TinkerTool.html