I would just tell him, I'd think of something such as 'I can't offer what you are looking for/feel under pressure/can't meet your demands'.
I'd probably tailor my words to the boundaries he's pushed. Because I tend to give warnings first, I can then refer to those saying I had tried, kept asking but no change.
Quite a few years ago a regular pushed boundaries every booking, be it money, time, free time, or services I was not keen on (and eventually decided to actually cease even trying those services). After much patience and trying (he had had plenty of free time/overruns/whilst enjoying a discount & trying those services) I finally explained it was either stick to time & stop pushing for things he knew I did not want to do/did not offer, or stop the bookings. He agreed but could not resist starting to sneak in the pushes again. I stopped accepting his bookings and told him why. Some months or even a year later he said he had changed, would not be pushing, wanted to have a booking. I agreed but it was the same so stopped again.
He has asked me again, I always ask him "Are you sure you will not push for....." I ask him very firmly, and he admits he won't stop pushing. Last time he contacted me I just said "No". I looked in CE there is a report which is exactly my experience even down to a few odd and repelling things about him. Whilst he was a regular he booked another Escort who told me just the same, at the time I had told him if he wanted certain services he was better booking someone who offered them. So I know it's not just me (which is easy to feel) being unreasonable. I seem to remember adding my own report to CE about him - making the timescale clear to show he does not change.