If the suggestion above doesn't work you can manually stop the process from the windows task manager. The open Geo Package uses Java stop if you kill all the java process on your system this will stop Jetty (the web container holding geoserver) from running.
I would only do it this way if nothing else works. Normally the reason that shutting down by another method doesn't work is because for some reason part of you configuration has be corrupted or changed so that the shutdown commands no longer connect to the correct port ect.
Shutting down this way does have a small risk of corruption to data and configuration ect.