Originally Posted by
enviroian
My SOP for this situation (it's happened several times)
1. Ask (politely) may I see your boarding pass
2. passenger shows me his pass and
3. If BP is a different seat I politely say wrong seat pal
4. If BP is the same seat I immediately reach up above his head to hit the call button and wait for flight attendant assistance.
The OP question was what to do in cases the person does not want to move, not how to ask.
For the initial interaction, FYI, personally, I don't like your approach. Some people may refuse to show their BP, like who are you? they don't need to show anything to anybody other than the FA.
My approach is to show my own BP, "Hello, good morning, I am on 8C -showing BP-I think this is my assigned seat". 99% of the times that prompts people to check their BP and correct the mistake. Or perhaps a friendly conversation and seat swap.
Rarely a person would refuse to move out of your assigned seat. It happened to me once though, when a toddler was on my assigned seat, the mother next to him said "I have to travel next to MY CHILD!!!". I waived at the FA who assigned me another aisle seat and gave me some miles on the spot on her tablet (This was DL though).