Answer by John Burnett: "Kiting" is a criminal offense, and it's never a good idea to accuse a customer of such an offense (or even suggest it). It can only backfire on you. There is actually no requirement that you provide a reason for closing a customer's account (unless there's a state law that calls for a reason). If your bank's management feels that it owes the customer something more than "your account will be closed," the letter could say "We have chosen to end our account relationship with you."
And if management is adamant about providing a more concrete reason, I suggest sticking with "Your account will be closed on [date] for mishandling."
Answer by Randy Carey: Or, "We have closed your account per our account agreement, paragraph X, which states that we reserve the right to close your account at any time for any reason".