How can I prevent Messages from opening?

Hello,

I was a user of the Messages beta on Lion, and while it had its flaws, I was mostly happy with how it operated. While it sometimes failed, it never bothered me unnecessarily.

Since upgrading to Mountain Lion, though, I find that Messages will open every time I receive a new message from my wife. I am not leaving the program open -- I quit after I send a message, and don't have an IM account set-up. I only ever use it to send a message, then quit.

This is annoying because Messages sometimes hangs when I open my laptop up, trying to catch-up and collect a conversation that occured on, say, my phone.

I'd like to keep the app on my computer, but would like to keep it closed unless I prompt it to open. I assumed there would be a preferences option for this, but if there is, I cannot find it.

Any thoughts on what I can do to prevent this behavior in the future?

Best,

John