Fired may be too strong a word but I had a situation when a client got behind (and behinder) in payments. (I have to explain that it is my practice to bill my clients monthly for the sessions they had; this has worked very well for me over the years.) She made small partial payments but continued to schedule sessions.
Before things got out of hand, I had to suggest to her that we suspend training until she was caught up with her financial obligations.
As it turns out, she eventually paid the entire amount she owed even though I had – in my mind – written it off.