Currently this is impossible to not have 'loaded' script. By loaded we mean that particular script file is set to be executed when shoot is pressed, ya?
At the moment the best idea is posted above:
@title default2.bas
shoot
exit_alt
end
'Unloading' script could be done very easily by small changes in menu and scripting routine (add a menu field and when the script is unloaded shoot button could just work as 'leave ALT'). Why nobody did that? Nobody found it usefull for him... You can try:) CHDK is opensource!