Must I pay my employee overtime when they were told not to work overtime in the first place?

If the employee is non-exempt and actually worked more than 40 hours that workweek (or 48 for certain employers in Minnesota), then unfortunately yes. This wouldn't necessarily preclude you from disciplining an employee for failing to follow well communicated workplace rules, such as "no working overtime," but the rule violation is not a legitimate reason to not pay them for the time they worked.

 
 
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