I don't think it's right to write them off their list. Not at all.

What they could do is to have an overall rule that states if you miss 3 appts. without cancelling, then you are on a call in basis. In other words, you call in and if they have an appt. open that day, you are given that appt. No making appts. in the future kind of thing. The offices that I know who do that, leave a certain amount of booking time for those cases and they find it works to keep the no call/no shows down. Of course, there are extenuating circumstances that should be taken into consideration as well, such as you mentioned.