My suggestion is that you close the account and then worry about filing the SAR.
Close it now.
Give appropriate notice citing a specific date when you will close the account. Reference language in your contract that allows you to close an account for any reason you please or simply say the customer is "consistently drawing on uncollected funds." That's politespeak for commiting bank fraud. Indicate if you will accept anything for deposit to cover outstanding checks; e.g. cash.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.