This article provides three ways of conveying that ad hoc hypotheses are bad science.
1) This video of a speaker lecturing about absurd logic behind ad hoc testing (here, evolutionary explanations for the mid-life "spare tire" that many men struggle with).
NOTE: This video is from an annual event at MIT, BAHFest (Bad Ad Hoc Fest) if you want more bad ad hoc hypotheses to share with students.
2) A quiz in which you need to guess which of the ad hoc explanations for an evolutionary finding is the real explanation.
3) A more serious reading to accompany this video is Kerr's HARKing: Hypothesizing after results are known (1998), a comprehensive take down of this practice.