Does anyone have any "generic" letter that they send to a customer requesting to close out an account at your institution due to suspicious activity going on?

Try something like this:

We've made a business decision to close your account. Please do not make any further deposits or write additional checks. We will send you a check for the balance in your account on ________. Any checks presented after that date will be returned "account closed."

If you have any questions please contact the undersigned at xxx-xxx-xxxx.