Is it possible to have an answer become unavailable to check if other answers are chosen when using the check boxes?
Add this last answer by clicking at Add “Other” / “N.A.” etc. (link at the bottom of the options page) and chose “None of the above”. After having inserted this option, you may replace the default option text “Non of the above” by your own answer text.
Result: the respondent can either click this answer option or any of the other ones, but not both.