Someone can be a bad fit for a number of reasons. Here are some examples. Feel free to adapt at will.
Clinical presentation you don’t enjoy:
“What you’re describing sounds really hard and I want to make sure you get the kind of care you deserve. Jane Doe actually specializes in what you’ve described and I know her to be a really great counselor. Since she has so much more experience in what you’re describing than I do, it feels more ethical for me to refer you to her. In case she’s full, I’m also going to recommend John Doe and Mary Doe. They also have extensive experience in this. Do you have a pen?”
Feeling creeped out by the caller and you already confirmed that you have availability:
“You know Jane, I’m getting the feeling that we might not be the best fit since I work with [describe someone different within your stated niche] and I don’t want to waste your time or money by having you come in for an appointment. I can think of a few people who might be a better fit for you, though. Do you have a pen?”
Demonstrates too-high-acuity-for-your-practice on first call:
“What you’ve described sounds really hard. I actually want to give you some referrals for people who I think will be better able to provide more comprehensive care than I can currently provide. I don’t want to provide anything less than what you need. I wouldn’t want to inadvertently make your life any harder right now. Do you have a pen?”
You’ll notice my “Do you have a pen?” for 2 reasons, first it’s a way of closing up the conversations gently but firmly. Second, it keeps you from getting into an email relationship with them. If they don’t have a pen, ask them if they can save the info into the Notes section of their phone. If they don’t have a smartphone, then offer to email it to them.