iMessage Question

Is there a way to delete a message history off your phone but still retain/save it somewhere for historical purposes (like you need to go back and pick out a piece of evidence that was said)?

The message file size on my phone is 1.8GB, but I think I found the problem. I have one message string to one person that's never been deleted; so far I've scrolled back ("Load Earlier Messages") all the way to Jul '13 and there's more. However, I really need to save this message chain. How can I get it off the phone to attempt to clear up some of this 1.8GB of Messages but keep this entire chain for access later?

I haven't confirmed, and don't know to, that this one chain is the culprit of the large file size but seems logical since it's full of pics and everything else.

...and I guess it's safe to assume when you have a string this long, even though you have to hit the Load Earlier Messages button, all this data is still actually on the phone?

 

I normally just use the cloud for all my normal backups, but here's a thought...

If I do a back up to iTunes and then delete the message chain in question off the phone but go back later and restore from that saved backup, won't that put that chain back on the phone?