If you [view] a message, there is an unlink button at the bottom for the author (and believe) and tracker gardeners.  I did this for the duplicate and incomplete messages.

In the future, lets put timing data for an idea on a separate issue for the idea.  This puts everything about the idea in a one place.  It is OK if we close the separate issue as rejected because the data show the idea not worthwhile.  

Unfortunately, there is no way to just move a message to another issue.  But it can be copied to another and unlinked from the first.