How to hide the Copy button on the Panel's button bar?

Is there a way to disable only the copy button on the Panel’s button bar of an Entity ?
The users need the Add (+) button but not the Copy button.

A typical case is on the BusinessPartner entity. They should not be allowed to copy a BusinessPartner as it will also copy the Username which should be unique.

Not really. But when you create define fields in entity design you have property OnCopyAction that might be set to Intitialize rather then Copy. This approach unfortunatelly cannot be changed if the entity does not belong to your model.

Then you could be able to “reset” the value by running State Workflow OnCreate event. There will be some transformation that lets you do whaterver is neccessary, e.g. omitting UserName filled or setting some value to the field.