That's the reason why I think that giving regular slots to clients and keeping them responsible for it (no cancelling policy) is good. It eliminates the possibility to act out this way. Sure, you can come or not come if you don't feel like but it is still your time, the T is there waiting for you and you have the responsibility to pay. I understand though that in US where the insurance system makes such arrangements difficult because the insurance people probably don't understand that missing such a session is also a therapy work.