I save them with a tag-number or description of their crime eg 'NS99', 'NSinameeting' and block.
I don't get repeat attempted bookings for no shows, cause they know they've been bad.
I also tag & block guys who irritate me, too impatient, abrupt or say insincerely, 'I'll ring later'...a couple might repeat ring, but it's not a stalking situation. I didn't used to tag all these annoying ones but I do now and feel better for it.