You need to remove the check-box on your finish page then?
You could delete it using:
!define MUI_PAGE_CUSTOMFUNCTION_PRE checkForProg
# check if it exists here?
DeleteINISec "$PLUGINSDIR\ioSpecial.ini" "Field #"
However, I don't know which Field number it is. It's probably something like Field 4... You'll have to experiment to get it right.
Obviously, if your program exists, then you don't want to jump over the DeleteINISec command. You can jump over it using StrCmp.
E.g. StrCmp $R0 "1" 0 +2