My method is very tailored to the vibe I get from the client. If we've met before, especially several times and he has a spat of no-shows, I don't hold it against him, I carry on in good faith that his reasons for cancelling last-second (or whatever may have happened) were legitimate and move on. An established client would need to make a habit of this behavior before I would take a precaution that would very likely damage our business relationship.
For clients that I have not yet met, I allow them two 'grace appointments' where we set up an engagement, agree upon a time and the person falls silent. After that, I demand a deposit of $50 (which is applied to the cost of the encounter) before I consider taking them seriously. At this point in my "career", I don't need the stress of someone reaching out to me last-minute then hearing back from an escort they contacted 4 hours ago and ghosting me.
Prior to enacting this policy, I had a following of approximately 20 guys who would reach out to be across a three-month or so time span and set up appointments only to fall silent. I have no way of attempting to divine why this was a pattern for so many people, as I have several friends in the industry and they report a similar problem but with a number of people closer to but less than 10. Anyways, since I started telling people this, it just doesn't happen.
I still get plenty of new clients, and they never learn about this policy unless they make a habit of failing to meet at the last second.
As a client, contacting a provider who just NEVER contacts back, I can't possibly make up an excuse for that behavior. It could only be a naive misunderstanding that your personal habits around clients is your brand. If you fail to meet someone, then you're damaging your brand in a way that could be easily resolved by even the flimsiest excuses. "Sorry, running late/flat tire/car exploded/girlfriend needed something/boyfriend needed something/personal emergency/developed a cough." the possibilities are endless.