Mac Tip: How to close all Finder windows with a keyboard shortcut
To get rid of all of them quickly, press Option+Command+W on your keyboard. Alternately, you can hold down the Option key and select “File” > “Close All” in the menu bar at the top of the screen. After that, all open Finder windows will disappear, lickety-split.
How to close all windows of Mac OS X Finder?