[Dr.Dave]

Normal mouse behaviour is to zoom with CTRL + Mouse-wheel. So you can "unzoom" by zooming in the reverse direction.

Press CRTL+0 to set the zoom level to 100%.

 

[Dr.Dave]

I'm not sure why this is a problem. I've been using Chrome a lot since day 1 of the original beta and I've never had the zoom level change by moving the mouse. I actually had to look it up to see how to do it with the mouse, since I always use the keyboard.

There isn't a switch to disable the zoom function, however there a number of extensions for zoom including one called Zoom Lock.