Stop/Pause iMessage updates in Messages app when closed?

Hey, all.

(This is specifically referring to the Messages app on the OS side...NOT the Messages app on iOS/iPhone/iPad.)

I'm loving having my iMessages accessible on my laptop/MacMini while I'm working or whatever. ...but, I've come into an annoying (and at times, embarrassing) discovery: Even if you close out of the Messages app, if you receive a new iMessage...the app will start itself up and display it for whoever is looking at your screen.

This is not good. When I'm at work, I don't want the Messages app popping up every few minutes when I get a text. ...especially when my boss is looking at something on my system, and my girlfriend decides to send me a "love" note. ... ... Yeah... Props at work, but I don't share. haha

How the **** do you get it to stop updating on the OS app when the Message app is closed?

I'm running systems with both Mountain Lion and Yosemite. Same experience on both. Help?

Thanks in advance!

-=R=-