For rules on advance notice, you're probably only bound by your account agreement (and possibly state law?), unless they have government benefits being deposited into the account, then you're looking at 30 days (I believe) notice.
Is it an account you've sent warnings to before? What are they doing that's significant enough to want to close their account, but you still want to be nice to them for?
Due to continuing high risk activity on your account, XYZ bank will be unable to continue doing business with you. Enclosed is a check for your account balance, please destroy your ATM/debit card, and discontinue use of any checks you may have.
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Someone's about to get horned!