I honestly do not know Chesty Morgan. The more I look it seems a number of people can including Cabin Crew and it seems of the Cabin Crew the PIC is at the top of the food chain and can override all other Cabin Crew.
BUT if a drug and alcohol tester walks on board and wants to test the PIC can he refuse or remove the tester? as he has ultimate authority prior moving under own power.
No he must comply with the request or order to the test, but not once moving under own power (if this tester is on a flight to another port and thinks the captain is pissed). The dispute then can only be followed up after the flight is complete as the captain does indeed have ultimate control.
If there were certain dramas on an aircraft and it required immediate response - yes I would act in the best interests of safety as I have certain knowledge of aircraft and systems. That does not imply I have the legal right to do so.