You wouldn't see this kind of behaviour on my flights. Recently, two passengers boarded with a big guitar each, in the kind of case that can also carry 3 week's worth of clothes. Both guitars had been tagged. Full flight. I simply said we'll leave the guitars in the front galley until fully boarded and if there's room at the end they can be stowed in the cabin. Worked a treat
Anyone else who tries to board over the limit I ask them if they're traveling on their own (as in whether they're the family pack-horse or not). If they're on their own, I remind them politely of the allowance and ask them which one they'd like to put in the hold. Not everyone is agreeable to it, but it does work.