For info, in BA, before between boarding and push back its the SCCM's decision to carry a passenger or not.
A decision to ofload a passenger at this stage cannot be overruled by the captain except he can offload if the CSD wants to carry.
Saying all that in my experience a quiet word usually does the trick and I've seen many many examples of gross over reaction by crew which has been easily smoothed over with a smiling threat between the teeth.
People usully quieten down when they think they won't get home.
If there is an investigation I hope the truth outs. If then its decided that the captain was guilty of a gross over reaction I hope the company get hung for it.
BTSM