I know you said you don’t want to change the form, but even a little? Convert the question to radio buttons –
they only allow one answer. And one question with round response devices instead of square boxes shouldn’t
confuse your users.
<i>Create an event that when someone clicks a checkbox, the event checks to see if another checkbox has been selected, or is in conflict, with the one previously selected, and then simply warns the user of the conflict.</i>