To stop XAMPP simply call this command:
sudo /opt/lampp/lampp stop
You should now see something like this on your screen:
Stopping XAMPP 1.8.2...
LAMPP: Stopping Apache...
LAMPP: Stopping MySQL...
If you get any error messages visit our community pages for help.
Also, note that there is a graphical tool that you can use to start/stop your servers easily. You can start this tool with the following commands:
sudo ./manager-linux.run (or manager-linux-x64.run)