I typically politely say something to affect of "Excuse me there is a line here."
The one time I had some do similar what the OP describes was while boarding in first class. I did say "Excuse me there is a line here." They starkly said, "I'm in First Class" to which I replied "We are all in first class." They got to the GA and handed them their board pass which beeped red. The GA told them to step out the line, at which point I handed the GA my BP which beeped green and proceed to board. But not before giving the perp the stink eye. I noticed when the perp finally boarded they walk to the back of the bus. They were low class.