Why is this so hard?
You should be having a yearly audit.
It keeps people honest.
Most of them are not big time thieves that went into the job looking to steal, some did but most are just regular people who borrowed a little to tide them over and discovered how easy it was. And they get hooked.
So just have a yearly audit and they won't do it.
“So just have a yearly audit and they won’t do it.”
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Or at least make it known amongst the staff that there WILL be periodic audits. That alone might discourage some employees from thinking about stealing.