If she put enough force into it, Michonne would likely be able to knock him out on the bar without doing permanent damage. She could also knock the stool out from under him and send him sprawling, possibly to hit his head against the floor and pass out from that too.
Or just choke him on an ice cube.
"I'm sure the world is weeping for your absence." Michonne wondered why she'd even thought this was a good idea. "Maybe if you stay quiet for a really long time you'll go back."