I've never had this issue - but I start managing expectations early on so that the parents who are shopping dont waste their time. When I pull out my manilla folder to plan the assessment with them, I tell them what I am looking for. If there is a concern about ASD, I make sure to say "because ASD is a concern, I have to rule mental ******ation (because no one uses ID outside of our field). And it can also be X, Y, or Z or normality, which is why I only do comprehensive evals." If I am getting a non ASD vibe from the kiddo, I do say "it's unlikely they have ASD because of (insert comment about joint attention/sharing of affect/eye contact, social overatures, etc) but my opinion doesn't matter (it does lol) because the data is more important. My testing will catch ASD if it there 99 times out of a hundred. But, this is why test - to get good objective information." I might also spend two minutes talking about why we test and diagnose (describe whats going, identify strengths and challenges, to predict, and to influence or treat (and the tx of ASD is supremely different than say ADHD."
This will filter the asd shoppers usually.