A network file on windows is different for different users
The only buffer is the RAM of the client computers: the previous instance may still be running if it has never been closed.
However, if they are using shortcuts, you should know that Windows Explorer can find the original (old) file and update the shortcut accordingly:
What could I do to force the users to use the latest file?
You should delete the old file.
However, if you are updating this file regularly, you should conisder using a "launcher" , for example: a simple powershell or batch file that will only be used to start the main executable. Then, instruct your users to start the program using this launcher.
This will allow you to edit the launcher file to reference the correct version when needed, something like that:
\\share\soft │ launcher.bat -> start ...\1.5\tool.exe │ ├───v1.0 │ tool.exe │ ├───v1.1 │ tool.exe │ └───v1.5 tool.exe