Messages won't force quit

The power button is the same button that you use for your fingerprint for Touch ID; you should be able to press it harder and hold it down. That isn't how you should normally shut down but it should work when other methods won't.

Try force quitting from Activity Monitor. Choose the process and then use the X icon in the upper left to force quit.

How to turn your Mac on or off - Apple Support

The Touch ID button is the power button

Turn off (shut down) your Mac

The best way to shut down your Mac is to choose Shut Down from the Apple menu .

Just as your Mac follows a startup process after it turns on, it follows a shutdown process before it turns off. That process includes automatically quitting all open apps and logging out of your macOS user account. 

If your Mac doesn't shut down, try Force Quit to close any apps that aren't responding. If that doesn't work, press and hold the power button ( Touch ID) until the Mac turns off.

https://support.apple.com/en-us/HT201150

I'll try to get some further help for you.

Glad it helped; I only responded because of macjack's notification looking for more information.

Let us know how Messages works when you restart it.

That's odd. Try holding the power button for about 15 seconds or until it shuts down