Applescript - Close all windows except frontmost window
If you're running it as an Automator quick action, then you can close all windows except the frontmost window of the frontmost application by using the code below inside a Run AppleScript action (you can delete any sample code that appears, including on run {input, parameters}...end run:

tell application id "com.apple.SystemEvents" to tell the first process ¬
    whose frontmost = true to tell a reference to windows 2 thru -1 to ¬
    if exists then tell the value of attribute "AXCloseButton" to click