On a few occasions when I've decided that I no longer wish to see certain clients usually due to them causing me stress that I don't want, I've first tried ignoring/saying I'm fully booked and if they have persisted I've just sent them a polite text saying something along the lines of "Sorry but I've decided not to accept any more bookings from you, wishing you all the best for the future though and hope you find what you're looking for, regards, Kat" Its mostly worked for me and they've got the message and not contacted me again, or they have replied saying sorry for whatever it is and thanked me for being honest. Its just basic manners really and being professional about it, just because *some* people don't have any I won't lower mine.