The objective part of the response is that you can put a stop to it by simply saying that a cashiers check cannot be used to purchase a cashiers check. There have been prior discussions on this topic and some BOLer's have made and enforced such a policy.
The subjective element deals with the customers' intent. You seem to believe they are using it as a bank account. Based on experience, I might agree with you if I knew a bit more about the facts. However, I also note that they could be using the technique to hide from their creditors. I've seen that happen.
Some examiners, one in particular in my area, believe the practice is indicative of money laundering or income tax evasion. Without additional facts that conclusion is nothing short of ridiculous, but he enforces it against the institutions he examines, threatening to write them up if they do not file a SAR. (He says they only have to file one, no continuing SARs are necessary even though one of the descendants of the original check is still outstanding. Hey, when you're making stuff up you can make up anything you want!)
Your best practice is to simply not to allow it to happen...
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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.