If you try to stop the policemen, you will be arrested at some stage. I am a little surprised that an airline captain is not aware of this, as airlines normally cover this type of thing in command courses.
Actually, despite my poor answer, I am.
The point I didn't make is that I will try and prevent the police entering my flight deck by asking them to conduct the breath test elsewhere (out of sight of the passengers). If they didn't agree then there would be a bit of a stand off. My point would be that it's a flight safety issue if I allow unqualified people who are unfamiliar with aircraft into the flight deck. Flight safety is where I have absolute authority, on the ground or in the air.