Yeah I'd use the "ignore but don't actually say I never want to speak to them again let alone see them" method. There is a
reason we don't want to speak to such people again let alone book them (usually because they have been pushy twats) and because of this pushy-twatness they are likely to change numbers or even don false accents to try and fool you into answering them again if you outright say never to call you again. If you don't actually say that their hope/delusion springs eternal and they never seem to change numbers or get the hint so you usually always know who is calling and who to avoid.
I had one very very creepy ( silence of the lambs voice he had
longest dragging hour of my life probably) persistent idiot finally manage to book me after I kept telling him on the phone I wouldn't see him and to stop calling. He changed numbers 4 times (!) and names twice and put on numerous false accents (yes really!) before finally getting me on an off day where I didn't recognise his peculiar speech pattern and whiny voice.
He was a horrid client, very creepy and caused trouble with the neighbours by shouting in the corridor outside when he was leaving because he didn't feel I had been affectionate enough