How to terminate.... sigh...

[mom-in-alberta]

I haven't had to do it yet, thank goodness.... but I would say "I regret to inform you that I can no longer continue care. Please read this, and if you have questions, we can discuss them at a later time. The last day of care is specified in the letter."
Simply and matter of fact. If they try to "but, but, but" just ask them again to read the letter and you will continue the conversation another time.
Good luck!!