This issue has been raised here a number of times... The "fix" is pretty simple: Establish and enforce a policy that a cashiers check can be 1) deposited or 2) cashed. It cannot be used, either in whole or in part, to purchase another cashiers check. There is no need for a letter, your tellers can either explain it or summon someone who can.
It's not like someone can make you sell them a cashiers check....
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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.