I understand that there is an inyan of a makom kavua, but part of that stringency is arriving early enough so as to ensure that the seat is unoccupied.
It is completely inappropriate to show up late to shul and make someone move, in essence being machmir on makom kavua but being VERY makel on other issues, such as embarrassing a guest and/or interrupting their davening. I believe that the term for such a person is a Chasid Shote.