Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
Your business is extremely important to us.
That is why we have allowed you to continue to charge purchases to your account in spite of your past due balance.
We cannot, however, allow this to go on indefinitely. We must insist that you contact us and let us know what is happening, or send us your payment to clear up your outstanding balance.
Until that time, your credit is suspended.
Sincerely,
[name]