not a model.
It sounds like your looking at the person leaving as a loss of a potential sell. But it wasn't a potential sell at all. It was just some one wasting your time. This happens all the time in business. Half of business is knowing when to cut bait and walk away.
If you have no standards, people will take advantage of you. And they'll just walk all over you. It's not worth it.
You should only do what your comfortable doing; because doing it is would be much worse.
My experience is if you make a big deal about it, they'll make a big deal about it. So honest, short and sweet, direct and to the point. It's not confrontational. It's just mater of fact, no. no anger, no excuses, Nothing to debate or question.
Some times it can be useful to keep the conversation moving. Maybe suggest a service you do offer that might be similar or related. Or just something you do do. Some people are going to be not interested and move on, but that's just how it works.
I don't do crushing, but I do offer balloon popping.
I don't do BDSM, but I do offer spankings.
I don't do that. But I do have a fap video for 200 tokens.