Is it ok to tell the customer we are closing their account due to structuring?

Why give them a specific reason for them to argue, especially one that accuses them of a crime? Just reference your deposit agreement which hopefully has verbiage saying you can close an account at any time for any reason and generically say, "We can no longer service your account."

I agree with Brian, but want to add a caution.

Many questions posted on BOL stem from audit or internal review findings relating to events that have already occurred, not the formulation of plans, policies, procedures, etc. for the present or future. If that is the case here (i.e., the question should have been "WAS it OK...?") then I'd dig a little deeper.

If "closing their account due to structuring" was the outcome of one or more communications with this customer, then I want to know every detail of what transpired and determine is there's any exposure to a charge that your staff aided abetted the crime (assuming ML actually happened.) Conversations meant to be helpful (at the time) can come back to bite you. Any discussion that might have lead to the customer organizing or timing transactions (to avoid reporting) can be construed as aiding & abetting...and the staff member(s) or bank can be sucked into the crime.