The script should not run unless you click the shutter while in alt mode. Alt mode always shows you the currently selected script, but that doesn't mean it's running, it just tells you which script would be run if you clicked the shutter. If it is actually starting just by going into alt mode, something is very wrong.
If things are working correctly, there is no need to "unload" a script, but you can go back to the default script by loading default.bas from the SCRIPTS directory.
If the script starts when you start the camera, then you have "script autostart" enabled in the script menu.