How do I delete contact suggestions from Messages in OS X?
Solution 1:
The answer provided by @Noldorin works for me:
Messages on OS X doesn't allow "Recent" contacts to be deleted, but Messages on iOS does allow this. An oversight by Apple, but this is a fix nonetheless.
To clarify, I looked at recent addresses on my iPhone/iMessage when starting a new conversation. There's an "i" symbol which allows your to see recent group conversations and addresses, and also a remove button.
Once removed the erroneous address is no longer visible in OS X iMessage.